The chief U.N. investigator leading the probe into the assassination of Lebanon's (search) former prime minister said Thursday he believed more people were involved in the killing than the five pro-Syrian suspects already identified.

Detlev Mehlis also said his U.N. Security Council-ordered probe into Rafik Hariri's (search) Feb. 14 killing in a massive Beirut bomb blast had not yet identified any Syrian suspects, but the German investigator added there had been "problems" with Syrian cooperation.

"The five [Lebanese] suspects we have arrested, in our assessment, are only part of the picture," Detlev Mehlis said at a news conference. "So we'll have to further investigate."

The investigation into Hariri's assassination took a dramatic turn with Tuesday's detention of three pro-Syrian generals who led Lebanon's security services when Hariri was killed but resigned in the ensuing political upheaval.

Another general, who is the current commander of Lebanon's Presidential Guards Brigade (search), surrendered for questioning, as did a pro-Syrian former lawmaker, who has since been released.

President Emile Lahoud, reacting to the arrest of the generals, vowed to stay in office and instructed Lebanon's judiciary to act on solid facts and evidence when dealing with the witnesses, three of whom are his close associates.

Lahoud "affirms to the Lebanese that he will continue to shoulder the responsibility in safeguarding the constitution and the laws," said presidential spokesman Rafik Shalala.

Lahoud's opponents began calling for his resignation before Hariri's assassination. The demands intensified after anti-Syrian groups swept to power in Lebanese parliamentary elections in May and June.

Mehlis also said he was willing to go to Damascus to meet Syrian officials and recommended to the Lebanese authorities that they keep holding the four generals in police custody in Beirut since Tuesday.

"The presumption of innocence stands," he said, adding the suspects had not been charged. But he said their detentions were based on evidence.

Many Lebanese blamed Syria and its Lebanese allies for Hariri's death. The claims have been repeatedly denied by the Syrian government and pro-Damascus Lebanese authorities.

Hariri's killing sparked massive protests here, led to the withdrawal of Syrian forces from Lebanon after 29 years and the ouster of this country's pro-Damascus government.

Mehlis said planning the bombing that killed Hariri, which the investigator previously declared was caused by explosives packed into a pickup truck, would have needed considerable time and more than five people to prepare.

The U.N. probe is expected to send shock waves through neighboring Syria by again raising suspicion that Damascus had a role in Hariri's slaying.

When asked about the cooperation U.N. investigators had received from Syria, Mehlis said that there had been "problems" but was "optimistic" that those problems can be solved.

"I am confident we will be able to get the necessary information from Syria," he added.

"It's clear that we didn't get the cooperation from Syria as fast as we would have hoped," he said.

Investigators want to interview Syrian officials who operated in Lebanon, and want those previously responsible for security here to stand as witnesses.

His interim report late August has already said Syria was "considerably" delaying his probe by refusing to turn over documents and ignoring interview requests.

Mehlis' three-month probe is nearing its Sept. 17 deadline and he's expected to report to the Security Council on his findings. He said he expects to request an extension to the probe.