Prosecutor-General Said Mirza told The Associated Press that he referred the four suspects late on Thursday to an investigating magistrate, who will interrogate them on Friday.

The four are Major-General Jamil Sayyed, the former chief of General Security; Major-General Ali Hajj, the former director-general of the Internal Security Forces; Brigadier-General Raymond Azar, the former director-general of military intelligence; and Brigadier-General Mustafa Hamdan, the commander of the Presidential Guard.

Mirza said the next step concerning the fate of the four generals will be decided depending on the results of investigating magistrate Elias Eid's interrogation.

Lebanese President  Emile Lahoud, for his part, vowed he would stay in office despite the arrest of the senior security officials, Aljazeera reported on Thursday.

More involved

Earlier in the day, the head of the UN probe, German prosecutor Detlev Mehlis, said he was willing to go to Syria to secure its cooperation in the investigation.

(L to R c'wise): al-Sayyed; al-Hajj;
Azar; Nasser Qandil; and Hamdan

Mehlis also said he believed there were "more people involved" in the February murder, following the questioning of five suspects, including the three pro-Syrian former security chiefs and Lahoud aide Hamdan.

Asked about Syria's cooperation so far, Mehlis told a press conference: "There were some problems but I'm optimistic that these problems can be solved."

"They will have to be solved because without this cooperation we will not have a full picture," he said, adding that he was "ready and willing to go to Syria."

Aljazeera quoted him as saying there were no Syrian suspects, adding that he would need an extension to his three-month term in order to wrap up the investigation.

Syria blamed

Lebanon's long-time power broker Syria and its political allies in Lebanon at the time have been widely blamed for having a hand in the killing, which plunged the nation into turmoil.

The four ex-security chiefs will be
interrogated again on Friday

Mehlis said the UN commission of inquiry had identified five suspects in the February murder and who had been "extensively interviewed". But he said they remained innocent until proven guilty.

"The five suspects we have arrested, in our assessment, are only part of the picture," Mehlis said

"We will have to further investigate and we do think more people were involved," he said.

"It is fair to say that the case is not closed and we still invite anyone inside and outside of Lebanon who may have information on this assassination to report it to us."