Saif Nazar, a 23-year-old Sunni who drove victims to Numan Hospital, said, "Some of them jumped into the river, even if they didn't know how to swim, just to escape the situation, so they drowned. Most of the people I rescued were women."
Gen. Rawad Rumediam, a military commander at the bridge, said that 3-foot-high concrete barriers put in place to prevent car bombers from entering probably contributed to the crush. Saadoun Dulaimi, Iraq's defense minister, said the checkpoints, where pedestrians were searched for explosive devices, may have slowed the flow of pilgrims across the bridge and contributed to the disaster.
"Crowds gathered and a scream caused chaos in the crowd, and the crowds just reacted and this sorrowful incident took place," he said at a news conference.
Brig. Gen. Abdul-Jalil Khalaf, military commander of Kadhimiya, said in a television interview that the bridge "was not suitable for the use of pedestrians."
Survivors, some wandering upon the bridge after the stampede, blamed security officials for the deaths. Many criticized the government of Prime Minister Ibrahim Jafari.
"We asked the army troops to lift the concrete barriers from the road but they told us that the Americans put them in and they can't move them," said Jasim Kinani, who was among the many black-shirted young male volunteers helping with crowd control around the shrine.
Many feared that bodies were still in the river. Health Minister Abdul Mottalib Ali said the government had been seeking to guard against possible violence, but added, "We never prepared for such a big number" of casualties.
The disaster is among the deadliest in recent Iraqi history. In the Middle East, stampedes in Mecca also have killed hundreds in recent years. In 1990, more than 1,400 Muslim pilgrims died during the annual hajj in Saudi Arabia.
Iraq's transitional government moved swiftly to limit potential political fallout, blaming "terrorists" for launching the mortar attacks before the stampede and suggesting that insurgents intentionally incited panic by spreading false rumors about an impending attack.
"We ask the people to be patient and vigilant and to continue our march for all of Iraq," Prime Minister Jafari said, declaring three days of national mourning.
"This tragedy will be a new reason to keep Iraq unified," he said.
U.S. Ambassador Zalmay Khalilzad said that Washington was "prepared to offer whatever assistance we can to help the victims of this terrible tragedy."
The deaths come at a particularly sensitive time for Iraq's ruling coalition of Shiites and Kurds, who, with Sunni Arabs and other groups, have been writing a new constitution that is to be put to voters in an Oct. 15 referendum. The draft faces opposition from many Sunni Arab leaders.
There also is growing frustration among Iraqis of all backgrounds about the government's inability to control violence and restore public services, such as electricity.
Opposition politicians lambasted the government for failing to provide adequate security and called for the resignations of Iraq's defense and interior ministers.
"This is going to weaken support for the government even more," predicted Saleh Mutlak, a top Sunni leader.
Despite the high death toll, most said the government would survive the crisis because Iraqis have grown accustomed to daily bloodshed.
"You have to realize how fatalistic the people have become," said Sharif Ali Hussein, leader of a monarchist political party. "The effect is cumulative. It's another crisis in the daily lives of Iraqis that will add to the malaise happening in the country."
Indeed, the day's irony was not lost on Shiites. After years of imprisonment and torture by a Sunni caliph, Imam Kadhim was poisoned and his body dumped on a bridge in the same area more than 1,000 years ago. Shiites consider it an obligation to cross the Aima Bridge before paying homage to Kadhim at the shrine.
"We are cursed," said one burly man on the bridge. "We are marked with mud."
Times staff writers Ashraf Khalil, Raheem Salman, Saif Rasheed and Shamil Aziz and a special correspondent in Baghdad contributed to this report.