Panicked by rumors of a bomber, thousands of Shiite (search) pilgrims broke into a stampede on a bridge during a religious procession Wednesday, crushing one another or plunging 30 feet into the muddy Tigris river. About 950 died, mostly women and children, officials said.

Hundreds of lost sandals littered the two-lane bridge while children floundered in the waters below, trying to reach dry land. The tragedy was the single biggest loss of life known in Iraq since the March 2003 U.S.-led invasion.

"We heard that a suicide attacker was among the crowd," said Fadhel Ali, 28, barefoot and soaking wet on the riverbank. "Everybody was yelling, so I jumped from the bridge into the river, swam and reached the bank. I saw women, children and old men falling after me into the water."

The crowd was on edge because of the 110-degree heat, a mortar barrage near the Shiite shrine where they were headed and the ever-present fear of homicide bombers, etched into memories after repeated attacks against large religious gatherings. Seven people died in the mortar barrage three hours before the stampede, the U.S. military said.

Police later said they found no explosives at the bridge — either on any individual or in any cars parked nearby. Instead, poor crowd control and the climate of fear in Iraq after years of bullets, bombings and bloodshed appeared largely to have caused the horrific carnage.

Marchers jammed up at a checkpoint at the western edge of the Imams bridge, which has been closed to civilians for months to prevent movement by extremists between the Shiite neighborhood of Kazimiyah and the Sunni district of Azamiyah (search) across the river.

"This tragedy was the direct result of terrorism; hundreds of innocent people, mostly women and children, have died because of the fear and panic that terrorists are sowing in Iraq," NATO Secretary-General Jaap de Hoop Scheffer said in a statement.

Defense Minister Saadoun al-Dulaimi (search), a Sunni, said three bombers were stopped Wednesday some distance from the shrine, but "blew themselves up before reaching their destination."

State Department spokesman Sean McCormack said in Washington that he was not aware of any evidence that the stampede on the bridge was caused by a bombing.

Others blamed the government and the U.S.-trained security forces.

"Early security measures should have been taken to protect the lives of citizens and organize their processions," Iraqi Communist Party leader Hameed Majid Mousa told Al-Arabiya television. "We all know that there are terrorists who lie in wait for such events and prepare to ambush the people. ... Why are the processions not organized?"

Iraqi President Jalal Talabani, a Kurd, told state-run Iraqiya television that "the government should take measures for an honest investigation to determine how failures doubled the casualties."

The marchers were commemorating the death in the year 799 of Imam Moussa ibn Jaafar al-Kadhim, one of the 12 principle Shiite saints who is buried in a mosque in the northern Baghdad neighborhood of Kazimiyah.

Since the 2003 ouster of Saddam Hussein, a Sunni, the Shiite political parties have encouraged huge turnouts at religious festivals to display the majority sect's power in the new Iraq. Sunni religious extremists have often targeted the gatherings to foment sectarian war, but that has not stopped the Shiites.

The ceremonies have often been chaotic, with huge crowds overtaxing the ability of police and security services to protect them. Television reports said about 1 million pilgrims from Baghdad and outlying provinces had gathered near the shrine on Wednesday.

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.

"Pushing started when a rumor was spread by a terrorist who claimed that there was a person with an explosive belt, which caused panic," Interior Minister Bayn Jabr said. "Some fell from the bridge, others fell on the barricades" and were trampled to death.

No official offered any evidence that Sunni insurgents were directly responsible for spreading the false rumor.

Scores of bodies covered with white sheets lay on the sidewalk outside one hospital under the broiling sun because the morgue was packed. Many of them were women in black gowns, as well as children and old men.

Sobbing relatives wandered among the dead, lifting the sheets to try to identify their kin. When they found them, they would shriek in grief, pound their chests or collapse to the ground, sobbing.

Prime Minister Ibrahim al-Jaafari, a Shiite, declared a three-day mourning period.

In other violence, a U.S. soldier was killed and three were wounded Wednesday when a bomb exploded in the city of Samarra, 60 miles north of Baghdad, the U.S. military said. The military also said another American soldier was killed Tuesday by a bomb in Iskandariyah, 30 miles south of Baghdad.

The several mortar and rockets fired at the Shiite neighborhood before the march struck about 600 yards from the Imam Kadhim shrine, the U.S. military said. U.S. Apache helicopters fired at the attackers.

In March 2004 homicide attackers struck worshippers at the Imam Kadhim shrine and a holy site in Karbala, killing at least 181 overall.

The head of the country's major Sunni clerical group, the Association of Muslim Scholars, told Al-Jazeera television that Wednesday's disaster was "another catastrophe and something else that could be added to the list of ongoing Iraqi tragedies."

"On this occasion we want to express our condolences to all the Iraqis and the parents of the martyrs, who fell today in Kazimiyah and all over Iraq," said the cleric, Haith al-Dhari.