Lebanon on Tuesday arrested three former pro-Syrian security chiefs over the murder of ex-premier Rafiq Hariri, the first major breakthrough in the probe six months after the attack that sent shockwaves through the nation.
The head of the presidential guard Mustafa Hamdan also turned himself in to UN investigators after Lebanon issued an arrest warrant against him.
The three arrested men - former general security chief Jamil al-Sayed, ex-military intelligence boss Raymond Azar and former internal security head Ali al-Hage - were also brought before the UN commission.
"These officials will be interrogated as suspects," Prime Minister Fuad Siniora said in a statement read on television.
Several other people were also arrested by Lebanese police and a warrant has been issued for former pro-Damascus minister and MP Nasser Qandil, who told AFP he was breaking off a working visit to the Syrian capital to appear before the commission.
They are the first major arrests in the probe into Hariri's killing, which plunged Lebanon into turmoil and increased the pressure on Syria to pull out its troops in April.
Justice Minister Charles Rizk said they were made at the request of the international commission headed by German prosecutor Detlev Mehlis investigating the massive Beirut bomb blast that killed Hariri on February 14.