A U.N.-backed investigator on Saturday reported "considerable progress" in his probe of a former Lebanese prime minister's assassination, and said most of his work could be wrapped up in several months.

The second report from Belgian prosecutor Serge Brammertz's investigators said Syria — which had earlier been accused of obstructing the probe into Rafik Hariri's death — has cooperated in a "generally satisfactory" manner.

"Syria has responded to all the commission's requests, and did so in a timely manner, and in some instances, comprehensive responses were provided."

Some senior-level Syrian officials have been implicated in the Feb. 14, 2005, bombing that killed Hariri, a quiet opponent of Syrian domination in Lebanon, and 22 others. Earlier reports from the team investigating Hariri's death have implicated Brig. Gen. Assaf Shawkat, Syria's military intelligence chief and the brother-in-law of Syrian President Bashar Assad.

Hariri's killing provoked an international outcry that ultimately forced Syria to withdraw thousands of its troops from Lebanon in April 2005, ending nearly three decades of military dominance of the country. Syria has denied involvement in Hariri's death.

Brammertz was expected to brief the U.N. Security Council on Wednesday — a day before his commission's mandate expires. Brammertz welcomed Lebanon's request that the Security Council extend his team's mandate for up to another year, though he said most of his work could be finished by sometime this autumn.

"This would provide a much needed sense of continuity and stability, guarantee progressive operations and planning, and offers assurances to staff," the report said.

France, a permanent member of the Security Council, is expected to back that request. In a cover letter accompanying the report, U.N. Secretary-General Kofi Annan thanked Brammertz and suggested that he would support a long mandate.

"I share the view expressed by Mr. Brammertz and the Lebanese government that the commission should be provided with stability and predictability in its mandate and resources," Annan wrote.

In the meantime, Brammertz' report gave the Security Council a rough outline of his investigation into the bombing that killed Hariri as his convoy moved through downtown Beirut.

According to the report, evidence collected so far suggested the bomb that killed Hariri was aboveground and fit with earlier theories that it had been packed into a Mitsubishi truck that was found demolished at the scene.

There has been speculation in Lebanon that there was a possibility of two explosions — one underground and another in a truck — especially after Brammertz's team recently erected a huge tent at the scene of the killing and dug into the ground.

"Evidence collected from within the soil on the inside of the crater, indicates that the IED (improvised explosive device) was most likely located above the ground," the report said.

Investigators are still trying to learn the identity of the possible bomber and whether he was coerced into doing it or was a willing participant. It said there was no evidence to suggest that a man named Ahmed Abu Adass, who appeared on a video tape claiming responsibility, was in fact involved.

Those details are important because they would help point to the perpetrators of the crime and how complex it was. Brammertz said he has two hypotheses: that a small team acting on its own planned and conducted the entire attack itself, or that it took a larger, complex operation, with a large number of people performing smaller, very specific tasks.

Brammertz' predecessor as chief of the investigation, Germany's Detlev Mehlis, had said the killing's complexity suggested the Syrian and Lebanese intelligence services played a role in Hariri's assassination. Yet Brammertz shied away from making any such claims.

The entire report was characteristically cautious and short on specifics. Brammertz has consistently refused to give any indication of where he is going with the investigation, a marked difference from Mehlis, whose updates read like detective novels and revealed tantalizing bits of evidence.

Richard Grenell, spokesman for the U.S. mission to the U.N., said American officials would have no immediate comment because they were still studying the 30-page report.

Four top Lebanese generals — key figures in Syria's domination of Lebanon — have been arrested and charged with playing a role the killing.

Much of Brammertz' work had involved combing through the evidence accumulated by Mehlis and filling jobs that had remained open for months, slowing the investigation. He said those problems had been largely taken care of, though he acknowledged there remained a shortage of language staff.

Hariri's team had also been asked to provide technical assistance to Lebanese investigators probing 14 other bomb blasts that hit in Lebanon since Oct. 1, 2004. In a report in March, Brammertz had faulted Lebanese officials for their lack of progress.

In Saturday's report, Brammertz said those blasts were probably connected. Yet he stressed that the Lebanese judiciary and law enforcement needed more resources to move those cases forward, and said his commission believes it could play a more proactive role.