Eyewitness: The weakest - including children and the elderly - were among the first to fall to their deaths from the overcrowded bridge, writes Rory Carroll in Baghdad.
For once, there were no cars rigged with explosives, no men with assault rifles or bomb-filled vests - no proof of malice.
The culprit was a combination of panic and the weight of Shia pilgrims crushing against each other.
One moment Ali Naji was part of a tense, solemn religious procession moving slowly across Baghdad's al-Aima bridge on his way to the golden domes of al-Kadhimiya shrine to commemorate the death of Moussa al-Kadhim, one of Shia Islam's 12 most revered imams.
The next moment he was part of a terrifying scramble for life in Iraq's worst disaster since the US-led invasion in 2003. At least 769 people were reported killed and 307 injured, but the final death toll could top 1,000.
"We heard some shouting," said Mr Naji (37), a civil servant, from his hospital bed. "People started pressing against us, but we couldn't move because there were so many in front of us.
"We couldn't breathe. My wife and children [ had] begged me not to go. They said there would be bombers."
People who had been in the middle of the bridge bore the brunt when panic set in, according to survivors interviewed at the children's central teaching hospital in al-Iskan.
The weakest - including children and the elderly - were the first to fall. Squeezed, fighting for breath, many fainted. Others stayed conscious but were knocked to the ground and clawed at the legs of those still standing, said Hadi Shakir (25), a street trader.
"They were crying, shouting out: 'Please rescue me!' But there was no way to help them. It was a matter of survival."
The fittest had the best chance of staying on their feet or fighting their way to the side of the bridge and jumping.
Mr Shakir's nephew, Hamid (12), lifted a trouser leg and showed teeth marks on his left calf. "I stood on faces and one of them bit me," he said. He survived because Mr Shakir hoisted him on to his shoulders and together they leapt about 12 feet on to the bank of the Tigris.
Those in the centre of the bridge had to jump at least 50 feet to escape into the brown river water. The strong among them swam to the banks, but others floundered and drowned. When a waist-high barrier broke, hundreds, including babies, fell in on top of each other.
One man who jumped was Fakah al-Hassan (41), a retired soldier. "I didn't want to die slowly," he said. "I wanted it to end fast." Someone hauled him on to the bank and he survived.
Some witnesses reportedly said that the panic erupted when an unknown voice shouted that a suicide-bomber was among the throng. Two hours earlier, mortars had landed near the shrine, killing seven people and wounding dozens.
"We hold the terrorists, Saddamists and radical extremists responsible for what happened," said Ammar al-Hakim, a leader of one of the Shia parties in government, the Supreme Council for Islamic Revolution in Iraq.
If that is confirmed - if the bodies being dredged from the Tigris were victims of a simple plot - Iraq plumbed a sectarian low yesterday.
But the evidence was sketchy. No one identified the alleged shouter. And if there was indeed one, it could have been a well-meaning pilgrim drawing attention to a suspicious character.
Not in doubt was the woeful organisation of the commemoration. It had been known for months that a vast crowd - some put it as large as a million - was due to descend on the 16th-century shrine.
It was clear beforehand that such a huge number of pilgrims would struggle to pass through the narrow alleys and souks.
Yet police and soldiers were thinly spread and focused on intercepting suicide-bombers. No one appeared to be in control, according to survivors who spoke to the media.
Slumped in hospital beds, they watched government officials pass the blame to insurgents, and some shook their heads. They felt that incompetence, not sectarianism, was the cause.
The only gleam in a day of darkness, said Mr Naji, the civil servant, was that Sunnis from Adhamiya district, which faces the shrine, rushed to help.
"They rescued people. They gave us water, food. They donated their blood."
If there is to be a Shia backlash, it is likely to be directed at a Shia-led government already blamed for shortages of electricity, fuel and clean water.
In the immediate aftermath, emergency services were overwhelmed by the disaster. It was left to ordinary people to try to save the injured and cover the dead.
By midday, reflexes honed by two years of atrocities were seen in the fleets of pick-up trucks and ambulances racing to hospitals.
A Shia militia, the Mehdi army, took over checkpoints, while government soldiers sealed off entire districts.
Three days of mourning were declared by the government.
The name of the imam commemorated yesterday, Kadhim, means "the enduring one". As the sun set over Baghdad last night, it was also a description of the city in which he lies.