The UN investigators, in their third report to the Security Council, said on Tuesday they were closer to a detailed understanding of how the plot was carried out and predicted success in getting to the bottom of the crime, in part because of better co-operation from Syria.
The new report was the first since Belgian Serge Brammertz took over the inquiry in January into the killing of al-Hariri and 22 others in Beirut on 14 February 2005.
But he gave far less information than his predecessor, Detlev Mehlis, the German prosecutor, who accused Syrian and Lebanese intelligence officers of involvement in the slaying and named a host of suspects.
Mehlis also accused Farouq al-Shara, the Syrian vice president, of providing "false information".
Co-operation with Syria improved in the last three months, the report said. Syria, which denies the charges, has waged a campaign to discredit the commission and Mehlis was refused an interview with al-Assad.
Nonetheless, the report said that "the commission is confident that its support to the Lebanese authorities will result in a successful outcome to the investigations within a realistic time-frame".
"A significant number of new lines of inquiry, identified since January 2006, have already enabled faster-than-expected progress in two important areas," the report said.
Brammertz, on leave as deputy prosecutor of the Hague-based International Criminal Court, has spent much of his time reviewing Mehlis' files to see if they can stand up in a trial, diplomats close to the investigation said.
Al-Assad has so far refused to
meet the UN investigators
He concluded that "the commission is closer to a more complete understanding of how the preparatory work was undertaken, how those who participated on the day performed their respective tasks, what those tasks were before, during and after the attack and of the overall modus operandi employed by the perpetrators for the attack".
The report referred to the killing as a highly complex "terrorist operation" and said those involved in carrying it out appeared to be "very 'professional' in their approach".
"They planned to a high percentage likelihood for success, and conducted the operation with high standards of individual and collective self-discipline," the report said.
"It must be assumed that at least some of those involved were likely experienced in this type of terrorist activity."
The commission said it had "laid the groundwork with the Syrian authorities for improved cooperation" and "looks forward to receiving timely and relevant responses to its requests".
Investigators were also still trying to establish that the Mitsubishi truck used in the bombing had been seen in Syria shortly before the killing and had been driven into Lebanon by a Syrian colonel, as stated in an earlier report.
Separately, Syria and Lebanon have promised to pay for the extensive fire damage done to Danish missions during the riots over the Prophet Muhammad cartoons in Damascus and Beirut in February, Per Stig Moeller, the Danish foreign minister, said on Tuesday.
Danish missions in Damascus and
Beirut were damaged in February
"Syria and Lebanon have informed me that they intend to reimburse Denmark for the damage done to Danish missions in Damascus and Beirut," Moeller said in a written reply to a question by the Danish People's Party deputy leader Peter Skaarup, a copy of which was posted on the parliament's website.
"The total cost of the damage is being finally worked out at the present time, after which details will be sent to the governments of Syria and Lebanon with a request for compensation," Moeller wrote in his parliamentary reply.