A UN investigation is closer to understanding the circumstances surrounding former Lebanese prime minister Rafik Hariri's assassination, and believes some of the perpetrators may have been involved in terrorism before, according to a newly released report. Chief investigator Serge Brammertz said his team would meet with Syria's President Bashar Assad and Vice President Farouk al-Sharaa in the coming month as part of its investigation into the Feb. 14, 2005, explosion that killed Hariri and 22 other people. He said Syria had promised more cooperation but he would wait to see whether it delivered. Brammertz's team said in the Tuesday report the blast was so meticulously planned that it probably was not the work of an inexperienced group. One theory is that a truck bomb killed Hariri, though investigators are also examining whether the bomb was buried in the road and detonated as his car passed. "The individuals who perpetrated this crime appear to be very 'professional' in their approach," the report said. "It must be assumed that at least some of those involved were likely experienced in this type of terrorist activity." Brammertz did not disclose many details about the investigation in his first report since becoming chief of the commission investigating Hariri's death. The lack of information was a marked contrast to the details delivered by his predecessor, Detlev Mehlis, who publicly and exhaustively described his theories about the explosion. Brammertz did not repeat Mehlis' conclusions that the killing could not have happened without the knowledge of senior Syrian and Lebanese intelligence officials. It was not clear if he disagreed with Mehlis or just did not want to discuss those details because of the sensitivity of the probe. Yet his intention to meet with Assad and al-Sharaa suggested he believed they could give valuable insight into Hariri's assassination. Assad had earlier resisted interviews, implying in newspaper interviews that he has rejected the team's requests on the grounds that he has international immunity. Syria denies involvement in Hariri's slaying and has said it is cooperating with the probe. At the same time, it has waged a campaign to discredit the commission. Hariri's death led to demonstrations against Syria and magnified the international pressure on Damascus to withdraw its troops, which it eventually did. The Security Council approved a probe into Hariri's assassination on April 8, 2005. The 25-page report said Brammertz's team is closer to understanding the exact circumstances of Hariri's assassination in part because it had analyzed unspecified forensic evidence that had not previously been studied. It said that one of the people killed in the blast had not been identified and was possibly a perpetrator. "Based on a systematic evaluation of available but previously not tracked forensic evidence, the commission has moved closer to establishing the exact circumstances and modus operandi of the blast," Brammertz wrote.