A U.N.-mandated investigation into the assassination of Rafik Hariri (search) has confirmed that a truck bomb killed the former prime minister, but it has not established whether it was a suicide attacker, the head of the inquiry said Friday.

Experts who examined the site of the Feb. 14 blast established "without any reasonable doubt" that the explosives were not buried in the street, as some have speculated, said Detlev Mehlis, a top German prosecutor.

"We are talking here about a probability of 99.9 percent" that the explosion was above the ground, he said.

Hariri and 20 others were killed when his motorcade was blown up in downtown Beirut — a bombing that set in motion mass protests in Lebanon and increased international pressure on neighboring Syria (search) to withdraw its forces. Damascus completed the pullout on April 26.

The Lebanese opposition accused Syria and elements in the Lebanese security services of killing Hariri, a charge both governments denied.

Some believed the explosives were buried underneath the street, which they argued would suggest the involvement of officials in the plot, since digging up the pavement would draw attention and require permits.

Asked whether the truck was parked or moving — suggesting a suicide bomber — Mehlis said he did not know and was trying to find out.

The explosives were most probably placed on a white Mitsubishi Canter pickup truck that may have had a gray tarp covering its load, he said.

He asked for the public's help, distributing pictures of a truck similar to the one in question and hotline numbers for anyone with information on the vehicle.

It was Mehlis' first disclosure on his work since his team of German and Swiss experts arrived in Beirut in late May.

An Arab TV channel on March 27 aired video by a bank security camera taken minutes before the explosion that killed Hariri showing a white Mitsubishi pickup truck passing six times slower than other vehicles before reaching the hotel.

Shortly after Hariri's motorcade was photographed entering the area, the camera's viewfinder is obscured by clouds of dust and debris, the apparent result of the huge explosion.

German police are examining the original tape, Mehlis said.

He called on any state or party to provide information to his commission, saying the commission would provide protection and confidentiality to witnesses who came forward.

"Any country which may possess such information ... and does not provide it to the commission will bear the responsibility should we fail in our efforts to establish the truth," he said.

Asked whether the commission may interrogate Syrian officials, Mehlis replied, "We will, of course, interview anyone who was in one way or another responsible for the security in Lebanon at the time of the crime."

The investigation is continuing as Lebanon holds staggered parliamentary elections, with a crucial round of voting in the north to come. Hariri's son, Saad Hariri, is campaigning against people he describes as pro-Syrian and remnants of the Lebanese-Syrian intelligence services.

The opposition has used Hariri's assassination in their campaigns in parts of the country, as it tries to wrest control of the legislature from pro-Syrians.

The U.N. Security Council in April authorized the investigation after a U.N. fact-finding team concluded that a Lebanese inquiry into the killing did not meet international standards. The team reported that evidence had been removed, ignored or planted.

Mehlis' team has three months from Thursday to work, though its mandate can be extended. Besides visiting the bombing site, the experts have been reviewing evidence collected by other investigations.

The Lebanese opposition has demanded — and won — the resignation of Lebanese security chiefs and the top prosecutor who are accused of at least negligence — or perhaps a coverup — in the local investigation.

The new justice and interior ministers, whose portfolios deal with the investigation, are pro-Hariri, unlike their predecessors at the time of the assassination.