The American Federal Bureau of Investigation (FBI) has been called in to investigate the devastating car bomb attack in the Iraqi city of Najaf which killed at least 95 people.
Hakim's death is a blow to Iraqi Shias
The FBI's top agent in Iraq, Thomas Fuentes said experts will look for links between the attack in Najaf and bomb attacks at the United Nations headquarters and Jordanian embassy in Baghdad, earlier in August.
On Sunday, tens of thousands of Shia Muslims joined the funeral procession of leading Shia cleric Ayatollah Mohammed Baqr al-Hakim, who was among those killed in the Najaf bombing.
The ayatollah's remains are being taken on a three-day funeral march before they will finally be interred in Najaf.
Four men have been arrested in connection with the bombing, outside one of Shia Islam's holiest sites, which also injured at least 200 people.
Local officials say the suspects include two members of the former regime from Basra and two non-Iraqi Arabs, reportedly Saudis.
'No shortage of suspects'
Mr Fuentes said the governor of Najaf, Haider Mehadi Mattar had requested the FBI join the investigation into the bombing.
He said agents will look for links with the truck bombing of the UN HQ on 19 August, in which 22 people died, and the attack on the Jordanian embassy on 7 August, which killed at least 14 people.
"We'll be obtaining samples if we can of the explosives used [in Najaf] and will be submitting that to laboratory analysis, and comparing it with samples taken from other bombings," Mr Fuentes told AFP news agency.
He said it would be at least another week before results of analysis of samples already taken from the UN and embassy bombings came back from Washington.
Mr Fuentes would not speculate as to who was behind the Najaf bombing but said there were plenty of suspects.
"Just start a list. Many groups are capable. There's no shortage of people [in Iraq] capable of conducting a bombing like this," he said.
In Baghdad, thousands of grief-stricken supporters of Ayatollah Hakim packed the streets as his coffin made its way through the city en route to the second holiest Shia city of Karbala, 80 kilometres (50 miles) south.
Mourners beat their chests and chanted calls for revenge as they followed the slow-moving truck carrying the coffin, guarded by men with automatic rifles.
The force of the blast was so great that only the ayatollah's hand was recovered from the scene of the attack.
Iraq's most senior Shia cleric, Ayatollah Ali Sistani, has blamed the US-led forces in Iraq for failing to prevent the bombing and called for a greater role to be played by Iraqis in establishing their own security.
Ayatollah Hakim's brother, Abdul Aziz al-Hakim, told mourners the occupation forces in Iraq were responsible "for all the holy blood that has been shed in Najaf, Baghdad, Mosul".
Tensions are running high as Iraqi Shias try to come to terms with what happened to the man who had returned to the country in May after spending more than two decades in exile in Iran.
Ayatollah Hakim was the leader of an Iran-backed group, the Supreme Council for the Islamic Revolution in Iraq (Sciri), and cautiously supported co-operating with the US occupying forces.