"There is an eye witness to the shooting who has given some information," Mohammed Ali al-Mardi told Reuters.

"We are hoping this will tighten the police's grip on the suspect or suspects."

John Granville, a 33-year-old officer with the U.S. Agency for International Development, was shot and killed while returning home from New Year's celebrations in Khartoum in the early hours of January 1. Granville's driver, Abdelrahman Abbas Rahama, 39, was also killed.

Granville was the first U.S. government official killed in Khartoum in more than three decades and U.S. security agents were looking into reports that a previously unknown militant group was behind the attack.

An Islamist Web site posted a message from a group calling itself Ansar al-Tawhid in Sudan (Companions of Monotheism), claiming responsibility for the killings.

Al-Mardi refused to comment on whether investigators were any closer to determining whether Granville and Rahama were targeted by a militant group, or whether they were attacked for other reasons.

"I will not release any information that might prejudice the course of the investigation," he said. The minister would also not say whether other witnesses had contacted police.

Local media on Monday reported the witness who had stepped forward was a minibus driver and that police were using his information to draw up an artist's sketch of the main suspect.

Sudan's daily Al-Sahafah newspaper added officers found two bullets fired during the attack on the roof of a nearby house.

U.S. agents from the FBI and the State Department's Bureau of Diplomatic Security flew into Khartoum last week to help Sudanese security services with the investigation.