Mark Benjamin, Salon.com, reports:

On Sept. 24, 2005, tens of thousands of protesters marched past the White House and flooded the National Mall near 17th Street and Constitution Avenue. They had arrived from all over the country for a day of speeches and concerts to protest the war in Iraq. It may have been the biggest antiwar rally since Vietnam. A light rain fell early in the day and most of the afternoon was cool and overcast.

Unknown to the crowd, biological-weapons sensors, scattered for miles across Washington by the Department of Homeland Security, were quietly doing their work. The machines are designed to detect killer pathogens. Sometime between 10 a.m. on Sept. 24 and 10 a.m. on Sept. 25, six of those machines sucked in trace amounts of deadly bacteria called Francisella tularensis. The government fears it is one of six biological weapons most likely to be used against the United States.

It was an alarming reading. The biological-weapons detection system in Washington had never set off any alarms before. There are more than 150 sensors spread across 30 of the most populated cities in America. But this was the first time that six sensors in any one place had detected a toxin at the same time. The sensors are also located miles from one another, suggesting that the pathogen was airborne and probably not limited to a local environmental source.

William Stanhope, associate director for special projects at the St. Louis University School of Public Health’s Institute for Biosecurity, has been closely following scattered government and news reports about the incident. He’s convinced it was a botched terrorist attack. “I think we were lucky and the terrorists were not good,” he says. “I am stunned that this has not been more of a story.”

The DHS scrambled for three days to confirm just what may have been in the air that day. On Sept. 27, it turned for help to the Centers for Disease Control and Prevention. The CDC did its own tests, and on Sept. 30 — six days after the deadly pathogens set off the sensors and well into the incubation period for tularemia — alerted public health officials across the country to be on the lookout for tularemia, the deadly disease caused by F. tularensis.

“It is alarming that health officials … were only notified six days after the bacteria was first detected,” House Government Reform chairman Tom Davis, R-Va., wrote in an Oct. 3 letter to Homeland Security Secretary Michael Chertoff. “Have DHS and CDC analysts been able to determine if the pathogen detected was naturally occurring or the result of a terrorist attack?”

Government officials say the sensors detected a natural event. “There is no known nexus to terror or criminal behavior,” Russ Knocke, spokesman for the Department of Homeland Security, told the Washington Post. “We believe this to be environmental.” “It is not unreasonable that this is a natural occurrence,” says Von Roebuck, spokesman for the CDC. “There are still no cases of tularemia.”

However, Salon has spoken to numerous people who were at the Washington Mall on Sept. 24. Four say they got sick days later with symptoms that mirror tularemia.

Relatively speaking, F. tularensis is an effective biological weapon. A little bit goes a long, deadly way. A tiny amount — 10 microscopic organisms — can cause tularemia. After an incubation period of three to five days (it can range from one to 14 days), tularemia attacks the lymph nodes, lungs, spleen, liver and kidneys. Symptoms include fever, chills, headache, muscle aches, joint pain, dry cough and progressive weakness. Left untreated, tularemia can kill 50 percent of those who’ve contracted it. Conventional strains of the bacteria do respond to antibiotics, reducing death rates to as low as 2 percent.

As with anthrax, the U.S. military weaponized and stockpiled F. tularensis in the 1960s. The Soviets are said to have engineered strains to be resistant to antibiotics and vaccines. A World Health Organization Committee in 1969 estimated that dispersal of 110 pounds of F. tularensis over a city of 5 million would incapacitate 250,000 people and 19,000 of them would die.

“The biggest concern is that a terrorist would use the organism because it has such a high infectivity rate with a low number of organisms,” says Dr. Steven Hinrichs, director of the University of Nebraska Center for Biosecurity.

Scientists have long said that if terrorists use tularemia in an attack, it will look like this: The bacteria will show up in the air in a city, rather than the country, and perhaps at a major event.

“If Francisella tularensis were used as a bioweapon, the bacteria would likely be made airborne so they could be inhaled,” the CDC warns in an information sheet on tularemia. In a June 2001 consensus statement titled “Tularemia as a Biological Weapon,” the American Medical Association warned an attack would come in “an aerosol release” in “a densely populated area.”

There is no evidence that terrorists have ever used tularemia as a biological weapon before, but it may have been used by the Soviets against German troops during the 1942 Battle of Stalingrad, according to a report by the Council on Foreign Relations. The report adds that microbe stocks in Russia, Kazakhstan, Georgia and Uzbekistan are insecure and terrorists could potentially steal weaponized strains of tularemia from them.

So far, there are no signs of a tularemia outbreak in the U.S. But because it comes on like the flu, it is unclear if the government would even know if a few people from the Mall that day scattered across the United States had tularemia. The amount detected in the sensors suggests a very small amount was in the air.

“Clinicians don’t often think of it, and it has a non-specific presentation,” says Jeff Bender, an infectious disease epidemiologist at the University of Minnesota. “It is basically flu-like symptoms that sound like every other disease you can get.”

Like anthrax, F. tularensis is a naturally occurring bacteria. It is typically found in small mammals like squirrels, water rats and rabbits, which is why tularemia has also been called rabbit fever. Those critters get it mostly from bites by ticks, flies and mosquitoes. People have contracted tularemia from insect bites or from handling or eating infected material or skinning dead animals. F. tularensis is a concern mostly in central and Western states, particularly Missouri, Arkansas, Oklahoma, South Dakota and Montana. Nearly all cases occur in rural areas, according to the CDC. Around 125 people in the United States get tularemia each year. Most cases in the United States appear to have come from insect bites or handling animals.

Although insects mostly transmit the disease, there have been cases where the bacteria appears to have become aerosolized in the natural environment. Bacteria from a dead animal could contaminate some soil. In the right conditions, the bacteria might stay viable in the environment for weeks. The soil might then get stirred up and cause the bacteria to be airborne. Fifteen cases of tularemia were reported in Martha’s Vineyard in 2000, apparently after lawn mowers or brush cutters stirred up contaminated material into the air. One person died. Public officials have theorized something similar happened in Washington: The bacteria got into the soil on the mall and it was the marchers themselves who kicked it up into the air.

It is unclear if such a scenario explains what happened on Sept. 24. “The fact that it happened in six locations would have supported an attack scenario,” says Hinrichs from the University of Nebraska Center for Biosecurity. Hinrichs has not seen any test results proving that what was in the air that day was a deadly pathogen. Still, he says that government officials would have to consider the incident as more than a natural event. “To have found it in all six would have raised their level of suspicion,” says Hinrichs. “It could be a failed attack.”

The sensors that picked up on the pathogen are part of the Department of Homeland Security’s Bio Watch program. Since Sept. 11, sensors have been placed in 30 of the most populated cities in the United States. Most cities have roughly 12 sensors, although Washington is thought to have more.

The exact locations of the sensors are a secret. Some are piggybacked onto existing air monitoring stations, used by the EPA to measure pollution. The sensors look for signs of the six pathogens scientists consider most likely to be used as biological weapons by terrorists, including F. tularensis. (Other pathogens include anthrax, smallpox and plague.)

Sept. 24 was not the first time the Bio Watch sensors had detected possible biological weapons pathogens. Since the system was deployed, sensors around the United States have identified pathogens that could be used as biological weapons on five separate occasions, Jeffrey Stiefel, program manager for Bio Watch chemical countermeasures, said at an open lecture at the National Institutes of Health on Oct. 6. In all of those cases, the detections were apparently the result of natural phenomena. Indeed, some critics have long worried that one weakness of the Bio Watch program might be the difficulty of distinguishing between natural events and terrorism.

In 2003, two Bio Watch sensors detected F. tularensis near Houston in what the government later determined was a natural event, though the environmental source was never identified. But this was the first time anything popped up in Washington. “This is the first time we have had a situation there that I am aware of,” says the CDC’s Roebuck. It is also the first time six sensors simultaneously picked up on the same thing. “It has never happened that way before — that many,” Stiefel of the DHS said in his lecture.

Just after the antiwar rally, DHS officials faced a perplexing situation. While the six sensors detected something, at first it was not clear what it was.

Filters are removed from the sensors usually every 24 hours. A laboratory then performs a preliminary test to look for signs of a deadly pathogen. Six filters from the Mall showed the existence of a possible pathogen during that first round of tests.

A second round of tests could confirm the presence of F. tularensis using polymerase chain reaction techniques, which detect DNA signatures. The second round of tests was conducted sometime between Sept. 25 and Sept. 27. But in the second round of tests, none of the samples from the filters was a full DNA confirmation that what was floating around Washington that day was definitely F. tularensis. But it looked like it could be.

“The collectors were concentrated along the Mall,” Stiefel said in his lecture. “That starts to say, ‘Something looks a little funny here. The bottom line here is that there is something out there.”

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This posed a quandary for department officials. Under the Bio Watch program, substances detected that are not confirmed positive pathogens can be ignored. But six sensors had detected the same thing in Washington during the biggest peace march in a generation. And Washington, D.C., is not exactly tularemia country.

There was another troubling thing. One of the sensors that went off was located at the Lincoln Memorial on the far western end of the Mall. Another was located near Judiciary Square, roughly two miles to the east and two blocks north of the Mall. A third was at the Army’s Fort McNair, more than two miles from the Lincoln Memorial down the Potomac River past the Mall, on the point of land where the Washington Channel and Anacostia River meet. The locations of the other three sensors have not been disclosed.

This makes a natural event on Sept. 24 more difficult to imagine. Under the government’s scenario, soil on or near the Mall somehow became contaminated with the bacteria, perhaps from the body or blood of a dead or injured small rabbit or squirrel. That soil then got stirred up — possibly by the marchers themselves — and floated across the Mall and beyond. Marchers and book festival attendees contacted by Salon say it was dusty on the Mall in the morning. But it rained early that day and stayed moist, making the dust theory perhaps less likely, at least after that rain.

“One sensor, I’d say maybe,” says biosecurity expert Stanhope of the dust theory. “Two sensors is a stretch. Six sensors? I’m sorry, you don’t have enough money to buy enough martinis to make me believe that it is naturally occurring at six different sites. I don’t think you could get me that drunk to believe that.”

As for how the bacteria may have erupted through natural processes, says Hinrichs of the University of Nebraska Center, “I can’t imagine how it could have happened.” Asked if he could imagine a scenario whereby F. tularensis could float around the Mall in the dust, Bender, an infectious disease epidemiologist, says, “Theoretically, it is possible.” Asked if it could have been an attack, he says, “The question you are asking, ‘Was this real or not?’ That is a very valid question.”

Another possibility is that somebody was testing U.S. biological weapons defenses. How sensitive are the sensors? How quickly and effectively can the government react?

“The Department of Homeland Security would have to consider the possibility that it was neither natural nor an attack, but that it was a testing of the system,” says Alan Pearson, a former DHS official, who is now the biological and chemical weapons director at the Center for Arms Control and Non-Proliferation, a nonpartisan organization. “Was somebody trying to see what would happen?”

Regardless of the source, Pearson says, he was troubled that it took the government nearly a week to alert the public. “It points out that the system is still not working fast enough,” he says. “If it turned out to be something that really affected people, which it turned out not to be, the system was too slow.”

The federal government says that the most compelling argument against a terrorist attack is that nobody got tularemia. That may be true. But some people say they caught something that day.

Mike Phelps, 45, says he attended the rally in Washington that day, traveling round trip by bus from Raleigh, N.C. On Sept. 27, he came down with a fever, sore throat and headache. Within days, he was coughing up dark phlegm. When he blew his nose, it would bleed. “It was gross,” he says. “I literally vomited out cup loads of phlegm. Most of it was dark-colored. I’ve never had anything like this before.”

Phelps’ doctor said he had pneumonia and prescribed antibiotics. A few days later, Phelps read about the tularemia scare and called his doctor. His doctor told him that if it was tularemia, he would have prescribed him the same antibiotics. Phelps says he called the CDC but was transferred to an automated system. Frustrated, he hung up.

Several members of the women’s peace group, Code Pink, also from North Carolina, who attended the march, say they got sick afterward. Stephanie Eriksen, a 46-year-old network engineer for AT&T, says she developed swollen glands and cold symptoms in her throat and chest. She developed a persistent cough that still lingers. “My throat has still not recovered completely,” she says. Eriksen says her 14-year-old daughter marched in Washington and got sick. She was tested for strep throat. Eriksen said the results were negative.

Aimee Schmidt, a Code Pink member and student at North Carolina State, says that she developed flu-like symptoms and a raging headache that lasted three days after the march. She says her eyes hurt and her whole body ached. She never went to the doctor. “I made a choice, wise or not, to just deal with it,” she says.

Of course, there are countless benign explanations for these symptoms. And it could be true that nobody got sick from F. tularensis on Sept. 24. But bioterror experts say that doesn’t prove it wasn’t a terrorist attack. The Japanese cult Aum Shinrikyo, they point out, made several unsuccessful biological weapons attacks before the sarin attack in the Tokyo subway system on March 20, 1995. Previous efforts by the cult to release a botulin toxin from a vehicle in 1990, and anthrax spores from a building in 1993, apparently failed to sicken or kill anyone because of faulty dispersal methods.

Terrorists may have made a similar screw-up in Washington on Sept. 24. “One of my working hypotheses is that there was an attack and they failed in their dispersion system,” says Stanhope. “They dispersed an incredibly low concentration.”

Government assurances that there is “nothing to see here” are reminiscent of the federal government’s initial response to the infamous anthrax attacks in fall of 2001. In an Oct. 4, 2001, press conference, then-Department of Health and Human Services Secretary Tommy Thompson emphasized that anthrax occurs naturally in the environment and that “there’s no evidence of terrorism.”

“I want everyone to understand that sporadic cases of anthrax do [naturally] occur in the United States,” Thompson said. Thompson said the first victim to fall ill, a Florida man, was an “outdoorsman” and that investigators were looking into a stream he may have drank from in North Carolina. That man, Bob Stevens, 63, died the next day from inhaling weaponized anthrax that was apparently sent to the offices of American Media Inc. in Boca Raton, Fla.

Soon after, anthrax was sent to the office of Sen. Tom Daschle, D-S.D. Government officials claimed it was a “common variety” and not the weaponized agent most feared. Of course, further investigation proved otherwise.

Currently, the investigation into what happened on Sept. 24 is ongoing. Government officials have apparently been taking soil samples around the Mall, attempting to pinpoint a natural source for F. tularensis. In the meantime, on Oct. 5, the National Institutes of Health announced it would award two contracts worth a total of $60 million to develop new tularemia vaccine candidates. The announcement said nothing of the events 11 days earlier.