They never reached home. In the first detailed account of the assassination, The Independent has learnt that when the car entered a roundabout, it was hit by machine-gun fire from one or more positions. Within seconds, the gunmen lying in ambush riddled the car with bullets and the men inside were dead or dying. Relatives say Iraqi security forces immediately sealed off the area and would not allow even an ambulance through.
The assassination was almost certainly the work of agents working for the Iraqi government. Baghdad has always feared the religious leaders of the Iraqi Shia, who make up about 55 per cent of the population, but who for centuries have been denied political power. In the past year, two other prominent Shia clerics have been killed and others attacked by gunmen in and around Najaf.
The government insisted that Mr Sadr be buried immediately with a minimum of mourning. But this was not enough to prevent the most widespread popular disturbances in Iraq since the Shia uprising in 1991, in the aftermath of the Gulf War, which almost overthrew President Saddam Hussein.
The scale of the outbreaks has become clear only in the past few days as witnesses reach Jordan and Iran.
The outbreaks happened because Mr Sadr, who for six years presided over his community with the tacit approval of the government, had gradually acquired a mass following among Shia youth, townspeople and tribal leaders.
Respected for his piety, he had become open in holding the regime - as well as the US and its allies - responsible for the miseries of the Iraqi people.
When his death was announced by the official news agency, demonstrations and clashes erupted throughout southern Iraq, where Shia are in the majority. In Baghdad, worshippers at a mosque in Saddam City, a vast slum, poured into the street, shouting: "God is great". The security forces immediately shot dead two brothers. Iraqi sources in Iran say 13 people died elsewhere in the city.
Some of the trouble began even before Mr Sadr was killed. In Nassariya, a Euphrates river town, the local governor had arrested the ayatollah's representative, Sheikh Aos al-Khafaji, two days before the assassination, for criticising the government. A local delegation went to the governor to have him released. When the governor refused, they attacked the city's security headquarters while shopkeepers went on strike. The government sent in heavily armed Republican Guards and declared a curfew. After the assassination there were more clashes, with demonstrators shouting: "Death to Saddam".
The worst violence occurred at a Shia shrine 20 miles from Nassariya. This may have appeared especially threatening to the government, as the shrine is close to the marshlands of southern Iraq, the redoubt of anti- government guerrillas. The security forces opened fire on demonstrators, killing at least five, including two 14-year-olds.
The death toll elsewhere is not known, but security forces are clearly under orders to fire at protesters immediately. Iraqis in exile in Iran say there were clashes in the Shia cities of Kut and al-Amarah on the Tigris, close to the Iranian border. They also report flashes of artillery fire near al-Basrah, the largest city of southern Iraq.
Iraqi security forces have also been arresting representatives of Ayatollah al-Sadr in Baghdad and in the Shia heartlands of the south. They have detained Akeel al-Mussawi and Sheikh Ahmed al-Shamki, who were part of his personal staff.
The last time Saddam conducted such a purge, in 1991, about a hundred Shia clerics disappeared without trace.
The well-planned purge suggests the assassination of Mr Sadr was only one element in a plan to break his movement. Laith Kubba, an Iraqi commentator living abroad, says: "After Desert Fox [the bombing of Iraq by the US and Britain] in December Saddam decided to eliminate all potential anti- government leaders in a pre-emptive strike to head off any uprising. Al- Sadr was the most visible of the Shia leaders."
This is possible, but two senior Shia ayatollahs, Mortadha al-Borujerdi and Mirza Ali al- Gharawi, were killed last summer. The latter was machine- gunned to death in his car with his stepson and driver in an attack similar to that which killed Ayatollah al-Sadr.
A deeper motive behind Saddam Hussein's decision to kill the Shia leader was probably Mr Sadr's increasingly hostile attitude to the regime and his growing political strength.
The ayatollah had built his strength over six years. He had a loyal corps of several thousand religious students. He appointed representatives in Shia areas. Tribal leaders (mostly Shia but some Sunni) came to him to have their authority confirmed. He appointed his own judges. His criticism of the government became more explicit.
Three weeks ago, the government's regional overlord asked him to tone down his criticism. He refused, though he must have guessed the likely consequences. In his last sermons, he seemed to foresee his fate, saying that when Iraqis heard of his death they should not cease their prayers.