The commander of the Presidential Guards, three former security chiefs and a former lawmaker are suspects in the assassination of former Prime Minister Rafik Hariri (search), Lebanon's prime minister said Tuesday.

Prime Minister Fuad Saniora (search) confirmed that the three pro-Syrian former security chiefs had been detained for questioning earlier Tuesday and that the guards' commander, Brig. Gen. Mustafa Hamdan, had been summoned to appear before U.N. investigators. A Justice Ministry statement said Hamdan had made the appearance.

U.N. investigators searched the security chiefs' houses, Saniora said.

The detentions were the first major police action since Hariri and 20 others were killed by a massive bomb in Beirut on Feb. 14. The three detainees have previously been questioned by Detlev Mehlis (search), the German prosecutor who is leading the U.N. Security Council-mandated investigation into the assassination.

The killing of Hariri, a billionaire businessman who was seen as quietly opposing Syria's role in Lebanese affairs, provoked massive demonstrations that led to Damascus withdrawing its troops from Lebanon in late April, ending a 29-year military presence in the country.

Many Lebanese blamed Hariri's assassination on Syria and pro-Syrian elements of their government. Syria and its Lebanese allies have denied any involvement.

Hariri, who was prime minister for 10 of the past 12 years, presided over Lebanon's reconstruction after the 1975-90 civil war.

Mehlis, the U.N. investigator, met separately with the prime minister and justice minister on Tuesday.

The Justice Ministry said the U.N. investigation had been granted permission to use the police to "carry out raids, searches and escorting of persons for questioning."

Lebanese security officials reported that former Justice Minister Adnan Addoum (search) had also been summoned to by investigators, but the ministry statement did not mention its former minister. The security officials spoke on condition of anonymity because of the inquiry's sensitivity.

Under Lebanese law, those held for questioning can be detained for up to four days, but the order may be extended.

Addoum and the three former security chiefs stepped down in April as calls mounted for their dismissal from Lebanese politicians opposed to Syria. Addoum was both prosecutor general and justice minister at the time of Hariri's assassination. A preliminary U.N. inquiry into the killing accused authorities of tampering with evidence.

The three detained were: Maj. Gen. Jamil Sayyed, the former chief of General Security; Maj. Gen. Ali Hajj, the former director general of the Internal Security Forces; and Brig. Gen. Raymond Azar, the former director general of military intelligence.

Mehlis and his team are questioning the officials in a hotel in the hilltop Monteverde area of Beirut.

According to the prime minister, Mehlis had said Nasser Qandil, a pro-Syrian former lawmaker, was wanted for questioning "as a suspect."

Police failed to find Qandil when they went to his house early Tuesday, but later in the day he returned from Syria and was met by police at Masnaa border crossing.

Qandil said at the border that U.N. investigators had asked him to appear for questioning.

"They wanted to see me, but they did not say I am a suspect," he said, adding he regretted that the prime minister had called him a suspect. Qandil said he would cooperate with U.N. investigators.

Saniora told reporters: "According to what I was told by Judge Mehlis, the questioning of these people will determine the next steps to be taken against them."

Many Lebanese have expressed fear about the consequences of the results of Mehlis' investigation, particularly if it blames people connected to Syria, which retains considerable influence in Lebanon. Last week, the United Nations accused Syria of refusing to turn over documents to investigators and ignoring requests for interviews.

Since Hariri's killing, a series of smaller bombs have exploded in commercial centers and cars, killing several people, including two anti-Syrian activists. The attacks are thought to be linked to tension between those who favor a Syrian role in Lebanon and those who oppose it.

Several prominent politicians and journalists have left Lebanon, apparently fearing assassination. Gibran Tueni, an anti-Syria legislator who is also an executive of An-Nahar newspaper, is in Paris where he was quoted as saying Lebanese officials had warned him he was on top of "a hit list."