CAIRO, Jan 3 (Reuters) - A U.S. security team will head to Sudan shortly to investigate the killing of a U.S. government aid officer and his Sudanese driver in a pre-dawn shooting attack in Khartoum on Tuesday, U.S. officials said.

John Granville, a 33-year-old officer with the U.S. Agency for International Development, was shot and killed by unknown assailants while returning home from New Year's festivities in a diplomatic vehicle shortly after midnight on Tuesday.

Both U.S. and Sudanese officials have been cautious about assigning motives for the attack, saying investigators were looking into all angles and it was premature to say whether the shooting was politically driven or a purely criminal act.

Washington has long had tense relations with Khartoum in large part due to the ongoing ethnic and political conflict in Darfur in Sudan's west, which U.S. President George W. Bush has labelled genocide, a charge the Sudanese government rejects.

The U.S. State Department said the security team would include officials from Diplomatic Security and the U.S. Federal Bureau of Investigation, and that they would collect evidence and interview witnesses.

"We are taking action to determine, working with the Sudanese government, who is responsible for these murders," State Department Spokesman Sean McCormack said, according to a transcript of his remarks at a news conference on Wednesday.

Sudanese media have reported that the attack took place in view of multiple witnesses. McCormack said that an initial team would be made up of people in the region, but they would be joined later by more people from Washington. The team was in the process of getting visas.

The U.S. embassy in Khartoum confirmed on Thursday it was expecting a team of investigators but could not say when they would arrive.

SUDAN SAYS ISOLATED INCIDENT

The Sudanese foreign ministry has described the attack as an isolated incident and said it was coordinating its probe with the U.S. side. Sudanese Foreign Ministry Spokesman Ali al-Sadig said Khartoum was committed to protecting diplomats it hosts.

Sudanese media have reported that Granville was struck by multiple gunshots fired by assailants who approached in another vehicle in an attack on a main street in Sudan's normally sleepy capital, where street crimes such as armed robbery are uncommon.

"Khartoum is one of the safest cities in Africa. This sort of violence is extremely rare," one Western diplomatic official told Reuters.

Asked if that meant he believed the attack had to have been politically motivated, he said: "You can draw your own conclusions."

The attack did prompt the U.S. embassy to issue a warden message urging its citizens in Sudan to "exercise heightened security awareness". The U.S. government said in August that it had information that "an extremist group" might target U.S. government interests or facilities in Sudan.

Sudan, which hosted al Qaeda chief Osama bin Laden in the 1990s, has been on a U.S. list of state sponsors of terrorism since 1993.

The attack came a day after a joint U.N.-African Union force took over peacekeeping in Darfur, where international experts say 200,000 people have died and 2.5 million been displaced since rebels took up arms against the government in 2003.

The plan is for the force to comprise 20,000 soldiers and 6,000 police, but current numbers are only about a third of those levels.

The shooting also came a day after Bush signed a law to make it easier for states, local governments, and mutual and pension funds to cut investment in companies doing business in Sudan, particularly its oil sector, due to the Darfur conflict. (Writing by Cynthia Johnston; Editing by Giles Elgood)

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