The head of a UN inquiry into the death of Rafik Hariri has said he believes more people than those questioned so far are connected with the killing.
Detlev Mehlis wants his UN mandate to be extended
Detlev Mehlis said those who had been detained were part of the picture.
But he said that further investigation into the killing of the former Lebanese prime minister was needed.
The suspects include three former Lebanese security chiefs and the head of the presidential guard, all of whom have close links with Syria.
Mr Mehlis said he needed Syrian co-operation, and was willing to go to Damascus to meet officials.
Many people in Lebanon blamed Syria for the killing of Rafik Hariri earlier this year.
Syria has denied any role, but has been criticised for hindering the United Nations investigation.
The killing of Hariri transformed the political landscape in Lebanon and led to the withdrawal of Syrian forces from the country.
Mr Mehlis' mandate from the UN Security Council ends on 15 September. He indicated that this would need to be extended if he is to complete the investigation.
On Wednesday, the Lebanese authorities freed one of the five men arrested and questioned in connection with the killings.
Nasser Qandil, a pro-Syrian former cabinet minister, had surrendered to a UN investigation team a day earlier and said he had co-operated fully. 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.