Abu Musab al-Zarqawi, a Jordanian national, was the focus of controversial pre-war US attempts to establish a link between Saddam Hussein and al-Qaeda.
Two months before Washington invaded Iraq, US Secretary of State Colin Powell told the UN Security Council that Saddam was linked to al-Qaeda - allegations that were disputed and hinged on Zarqawi, a veteran of Usama bin Laden's training camps in Afghanistan.
Now Jordanian and US officials claim the 36-year-old Zarqawi is their chief suspect in Thursday's embassy bombing which killed 13 people and wounded more than 50 in the deadliest attack in Iraq since Baghdad's fall on 9 April.
Zarqawi, whose real name is Fadel Nazzal al-Khalayleh, fled to Iraq in 2002 and is accused of working with a Kurdistan-based Islamic guerrilla group, Ansar al-Islam, which Washington claims has links with al-Qaeda.
"The style of the attack and the explosives used, point towards Ansar al-Islam and in particular to Zarqawi, who is still on the run in Iraq."
"The style of the attack and the explosives used, point towards Ansar al-Islam and in particular to Zarqawi, who is still on the run in Iraq," a senior Jordanian official said on condition of anonymity.
"Zarqawi blames Jordan for giving the United States information on his terror network and a secret Ansar al-Islam camp in northern Iraq, which makes him one of the chief suspects in this attack," the official said.
US forces conducted a devastating air and ground assault on Ansar bases in northern Iraq in April amid accusations that the group was working with Baghdad and al-Qaeda to develop weapons of mass destruction.
The accusations against Zarqawi came as Washington resumed its wartime mantra that al-Qaeda had sought to acquire chemical and biological weapons from Saddam Hussein.
A report released by the White House on Friday cited a senior al-Qaeda detainee as saying that Zarqawi had come to Baghdad in May 2002 for medical treatment, along with two dozen al-Qaeda militants.
"This group stayed in Baghdad and other parts of Iraq and plotted terrorist attacks around the world," the report said.
Others suspect Iraqis who had a score to settle with Jordan over its decision to grant asylum to Saddam's daughters
Pentagon officials confirmed on Thursday that the US military was focusing attention on the Ansar al-Islam in its own inquiry into the embassy bombing.
Last weekend, US civil administration Paul Bremer said he had "clear evidence of an al-Qaeda related terrorist group, the Ansar al-Islam, reconstituting its capabilities inside of Iraq since the war."
Although the US and Jordan are pointing the finger at radical Islamist groups, others suspect Iraqis who had a score to settle with Jordan for a host of reasons, including the Hashemite kingdom's decision to grant asylum to Saddam's two daughters one week ago.
An Iraqi newspaper reported Saturday that a group calling itself the Humanitarian Aid Forces for Iraq had handed out pamphlets at the beginning of the week calling for anti-Jordan demonstrations by the embassy.
The group said it was angry at Amman for the treatment of the thousands of destitute Iraqis living in poverty in Jordan.
On Thursday, a crowd had mobbed the embassy after the explosion and destroyed pictures of King Abdullah II and his father, the late King Hussein.
At the site of the blast on Saturday, debris and four metal car frames lay on the ground guarded by Iraqi police, while US soldiers were positioned inside the compound.
The Iraqi interior ministry has asked the US Federal Bureau of Investigation (FBI) to take charge of the probe into the blast in, a coalition spokesman said on Saturday.
"The Iraqi ministry of the interior has requested assistance from the FBI," the spokesman told AFP on condition of anonymity.