UN investigators probing the February killing of the former prime minister that transformed Lebanon's political landscape named the suspects on Tuesday and interrogated all five.
Those suspected are a top aide to President Emile Lahoud, three former security chiefs and a former member of parliament close to Damascus.
The UN team handed them overnight to Lebanese authorities, who kept them at police barracks under heavy guard.
However, the ex-MP - staunch Syria ally Nassir Qandil - was freed in the early hours while documents detailing the interrogation of the other four men were sent to Lebanese Public Prosecutor Saeed Mirza.
Security sources said a magistrate, Judge Elias Eid, was questioning the suspects before he decides whether to issue arrest warrants against them, extend their detention or free them. He has at least 48 hours to rule.
Syria allies implicated
The detentions were widely seen by Lebanon's media as a breakthrough in investigations, with the UN team implicating for the first time allies of Syria in the bombing that killed al-Hariri and 20 others in Beirut.
The UN Security Council ordered the inquiry after a fact-finding mission found Lebanon incapable of carrying out a credible investigation into the attack that hastened the departure of Syrian troops from the country after three decades.
It welcomed Tuesday's detentions and again urged all parties to cooperate with the UN investigator, veteran German prosecutor Detlev Mehlis, and his team.
"I am always prepared to give all the probe requires in its actions, if the team believes we have something that would benefit the investigation"
former Lebanese MP
The US said Syria, which many Lebanese blame for the murder, was still not fully cooperating with the inquiry.
Damascus denies any role in al-Hariri's death and has pledged to work with the inquiry.
Mehlis, who will hold a news conference on Thursday to outline the probe's development, was expected to reveal Syria's response to his requests of cooperation and the inquiry's length.
The Security Council gave the German prosecutor a renewable three-month mandate to investigate the killing. He began work on 25 May.
Former MP Qandil said he was freed hours after he handed himself in to the UN team.
"I am always prepared to give all the probe requires in its actions, if the team believes we have something that would benefit the investigation," Qandil said.
Lebanese police detained Major-General Jamil al-Sayyed, former head of General Security; Major General Ali Hajj, ex-chief of police; and Brigadier-General Raymond Azar, former military intelligence chief, in dawn raids on Tuesday at the request of UN investigators.
Qandil and Brigadier-General Mustafa Hamdan, the head of Lebanon's Republican Guard, handed themselves over to the UN team.
Hamdan, a close aide of pro-Syrian Lahoud, was the only security chief to keep his job after parliamentary elections that ended in June ushered in an anti-Syrian presence majority for the first time since the end of Lebanon's 1975-1990 civil war.
UN investigators, backed by Lebanese police, also raided several flats in Beirut and its outskirts, witnesses said. It was not immediately clear what they were looking for.