CNN reports:

BAGHDAD, Iraq (CNN) — Deaths mounted steadily in northeast Baghdad after a massive midday Shiite religious procession erupted into a chaotic stampede Wednesday, causing the drowning and trampling deaths of 965 pilgrims.

Authorities believe a rumor raced through the crowd that a suicide bomber was in their midst, and that created panic among the waves of pedestrians trying to cross the Al-A’imma bridge over the Tigris River. The throngs of Shiite faithful had been stopped by security checks and bogged down by concrete barriers.

Three hours earlier, an insurgent mortar attack near the Kadhimiya mosque killed seven people and wounded 36 others.

Government officials are investigating that attack and the stampede itself — which also led to the injuries of 465 people. The death count could rise as crews search for more victims.

They also want to explore the extent of any “technical defects” on the bridge.

“This will leave a scar in our souls and will be remembered with those who died in the result of terror acts,” said President Jalal Talabani, echoing the sorrow across the nation over the tragedy, which prompted comparisons to stampedes at other religious events, such as those in Mecca during the annual pilgrimage in Saudi Arabia.

Wednesday’s ceremony is one that annually attracts millions of Shiite pilgrims to Baghdad.

The Shiite faithful converge on the Kadhimiya mosque in the northeastern part of Baghdad to commemorate the martyrdom of Imam Moussa al-Khadhem, a prominent figure in Shiite history. He is buried at the Kadhimiya mosque, the largest Shiite mosque in the capital.

The stampede occurred at about 11:30 a.m. (3:30 a.m. EDT) near the mosque.

As the people made their way to the mosque, authorities said, it appeared that someone in the crowd stoked fears about a bomber, apparently sparking widespread chaos among the crush of people.

In the confusion, the crowd pushed against a railing on a bridge over the Tigris River, forcing it to collapse and sending dozens into the water. Police said most people drowned but some people were crushed to death. (See video of the bridge still jammed with people after the panic.)

Health Minister Abdul Muttalib Ali confirmed that that “the chaos that happened at the Al-A’imma bridge” because of a “rumor” of a bombing. “This led to a very horrible chaos,” the official said, causing people “to run in an uncontrolled way and this led to suffocation of so many people and drowning of some of them in the river.”

Defense Minister Saadoun al-Dulaimi told reporters that there were concrete barriers on the bridge and people moving forward on the structure had to be searched “for security concerns.”

“There had to be a search operation at the end of the bridge. So crowds gathered and a certain scream caused chaos in the crowds and the crowds just reacted and this sorrowful incident took place.”

Maj.-Gen. Jawad al-Daini, commander of the al-Rusafa sector of eastern Baghdad, said barricades and barbed wires made it hard for people to walk and “that caused a congestion and suffocated some of the visitors.”

Prime Minister Ibrahim al-Jaafari — who announced a three-day mourning period — tried to calm his beleaguered fellow citizens in a speech that addressed both the mortar attack and the stampede.

Speaking in a nationally televised TV address, he tried to calm fears and tempers, urging Iraqis “to be patient with the current circumstances” and called on the country’s Shiites, Sunnis and Christians “to think about the benefit of Iraq.”

“We heard the news that some of those enemies attacked innocent people with mortars killing our faithful sons. This attack caused chaos among the Iraqi people in other areas in Baghdad and then because of the technical defects of the bridge, many people were martyred.”

Al-Jaafari also offered his “gratitude and appreciation to the Iraqi people who continued with their commemoration despite what happened and I want to thank the Iraqi government who helped the Iraqi people and did not stop providing their assistance and security to the people.”

The tragedy came amid an atmosphere of general tension in the capital and throughout the society — daily warfare across the nation, suicide bombings that had been rife in the capital, the continual tensions between Sunni Arabs and Shiite Arabs, and an attack in the same area in March 2004 during the Ashura commemoration, another Shiite holy period.

Al-Dulaimi — who spoke at a press briefing later with Interior Minister Bayan Jabr — stressed that the security had been tight in Baghdad and in southern cities around the so-called Triangle of Death for the Shiite pilgrims heading to the capital.

They pointed to a number of security actions that helped keep order, such as the foiling of car bombings and suicide attackers and the disarming of improvised explosive devices. In one case, an Afghan insurgent headed toward Kadhimiya was killed.

Al-Dulaimi believes all necessary measures had been taken to protect the pilgrims.

The bridge across the Tigris, called the Al-A’imma, is not only a main thoroughfare leading to the shrine, it is also an important juncture in Baghdad — separating Kadhimiya and Adhamiya.

The largest Shiite mosque in Baghdad is in Kadhimiya, where there is a strong Shiite community, and the largest Sunni mosque in Adhamiya, which has been a longtime insurgent stronghold with a strong Sunni Arab presence.

Al-Dulaimi discounted any theories that this incident was sparked by Sunni-Shiite sectarian violence.

He and other officials thought the bridge should have been closed. Instead, it was opened to accommodate the throngs of pilgrims.

The bridge — which frequently has been closed to vehicle and/or pedestrian traffic for security reasons over the past few months — is also near a joint U.S.-Iraqi military base.

– CNN’s Kianne Sadeq, Cal Perry, and Enes Dulami contributed to this story

Copyright 2005 CNN. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed. Associated Press contributed to this report.