Mr Qandil is a close ally of Syria
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Lebanon has freed a pro-Syrian figure questioned about the killing of ex-Prime Minister Rafik Hariri, but four other suspects remain in custody.
Nasser Qandil, a former cabinet minister, had surrendered to a UN investigation team a day earlier and said he had co-operated fully.
Three former senior Lebanese security officials and the presidential guard commander are still being questioned.
A magistrate will decide whether to order their detention or free them.
"I am always prepared to give all the information the probe requires," Mr Qandil said
"We want the truth, not like those who are trading the blood of the prime minister for small goals."
Those still being questioned are:
- Maj Gen Jamil al-Sayyad, former head of General Security
- Maj Gen Ali Hajj, former chief of police
- Brig Gen Raymond Azar, former military intelligence chief
- Mustafa Hamdan, Republican Guard Commander
All five have close ties to Syria.
Many Lebanese blame Damascus and its allies for the car bomb that killed Hariri in February. Syria has denied any role, but has been criticised for hindering the United Nations investigation.
The detentions constitute the first major development in the investigation.
"This is the start of the process of uncovering the truth and other arrests will follow," MP Saad Hariri, son of the former prime minster, told French radio.
UN Report
Lebanese President Emile Lahoud said the men being held were innocent until proven guilty.
"No one was arrested and these people are only being questioned," he said
The chief UN investigator, German prosecutor Detlev Mehlis, is due to report his findings to the Security Council in the next few weeks.
He has questioned all of the men before except Mr Qandil
The three security chiefs had quit in the aftermath of the bombing, but Mustafa Hamdan, a close aide of pro-Syrian President Emile Lahoud, has refused to step down.