US authorities in Iraq say the suicide bombing that killed the head of the Iraqi Governing Council bears the "classic hallmarks" of al-Qaida-linked Jordanian extremist Abu Musab al-Zarqawi.
However, General Mark Kimmitt says investigators are also probing an unknown group, The Arab Resistance Movement, which claimed responsibility for killing the Iraqi leader.
Izzidin Salim died earlier on Monday in a suicide car bomb attack in Baghdad. Six other civilians were also killed in the massive blast.
President Bush condemned the killing as a "brutal act of terrorism, but said it will not delay the planned June 30th transfer of sovereignty in Iraq.
A civil engineer and Sunni Muslim from the northern Iraqi city of Mosul, Ghazi Mashal Ajil al-Yawar, was quickly sworn in to replace Mr. Salim.
Meanwhile, US military officials say a small amount of the deadly nerve agent Sarin was recently found in an artillery shell that exploded near Baghdad. The shell went off before it could be defused, slightly injuring two US soldiers who were treated for what officials called "minor exposure."
Defense Secretary Donald Rumsfeld said more testing will be done to confirm the agent is Sarin. If confirmed, it would be the first discovery of banned weapons in Iraq since the US-led invasion last year.
Ousted President Saddam Hussein said his government destroyed all such chemical ordnance before the 1991 Gulf War.
Meanwhile, two Russian contract workers abducted last week south of Baghdad were freed unharmed in the capital. A third Russian was killed in the May 10th kidnappings.