The death toll rose Thursday, as Iraq's Interior Ministry said that the number of people who died now stands at 953. Another 815 were injured.
Most of the pilgrims in the accident — predominantly women and children — were trampled to death on the Two Imams bridge, although some jumped or were pushed into the muddy Tigris River about 30 feet below and drowned, officials said.
It was the single biggest confirmed loss of life in Iraq since the March 2003 invasion.
Tensions already had been running high in the procession in Baghdad's heavily Shiite Kazimiyah district because of a mortar attack two hours earlier against a shrine where the marchers were heading. The shrine was about a mile from the bridge.
CBS News Correspondent Lara Logan reports that Iraq's Deputy Prime Minister Ahmad Chalabi said someone shouted there was a suicide bomber in the crowd. Even though no bomb exploded, the fear alone was deadly as hundreds were either crushed in the panic or drowned in the Tigris.
Reflecting the confusion, casualty figures from various government agencies also varied widely. The Health Ministry said 769 people were killed and 307 wounded, while the Interior Ministry put the figure at 844 dead and 458 injured. The country's biggest Shiite party gave figures of 759 dead and 300 wounded. Other reports estimated the death toll would climb above 1,000.
In other recent developments in Iraq:
Prime Minister Ibrahim al-Jaafari, a Shiite, declared a three-day mourning period after the tragedy.
Defense Minister Saadoun al-Dulaimi said three suicide bombers were stopped Wednesday some distance from the shrine, but "they blew themselves up before reaching their destination."
Dr. Hamid Jassim, the head of a medical team accompanying the pilgrims, said some people did go off the bridge at the start of the stampede, but the crowds soon started pressing in both directions, and "most of the casualties either died from suffocation or from being trampled,"
"Many of the panicked people who jumped into the Tigris trying to save themselves survived with broken bones. Others drowned because they did not know how to swim," he said.