BEIRUT (Reuters) – A United Nations investigator intends to question Syrian officials directly as part of a probe into the killing six months ago of Lebanese former prime minister Rafik al-Hariri, a U.N. official said on Saturday. Detlev Mehlis will also probably ask for more time than the designated three months to complete his findings, the official said. “Detlev Mehlis needs to directly interview Syrian officials concerned. He needs to visit Syria for this purpose,” U.N. spokesman Najib Friji told Reuters. “The Syrians have agreed in principle to cooperate with Mehlis but he has yet to receive an official Syrian response to visit the country.” Mehlis, a veteran German prosecutor, is leading a 50-member team investigating the February 14 bombing that killed Hariri and 20 others in Beirut, throwing Lebanon into its worst crisis since the end of the 1975-1990 civil war. Many Lebanese hold Syria, which controlled Lebanese politics and security in the 15 years following the end of the civil war, at least indirectly responsible for Hariri’s killing. Damascus denies any role but withdrew its troops from Lebanon in April, ending a 29-year military presence amid mass anti-Syrian street protests and intense international pressure. The U.N. Security Council ordered the investigation, which began in mid-June, after a U.N. fact-finding mission found Lebanon’s own inquiry to be “seriously flawed.”

It was not clear which Syrian officials Mehlis plans to question, although he announced at the start of the inquiry that “we will … investigate anyone who was in one way or another responsible for security in Lebanon at the time of the crime.”

Lebanon’s As-Safir newspaper reported on Saturday that Mehlis had already questioned three Syrian officials in writing rather than in person after Damascus declined direct interviews.

Friji denied Syria’s former security and surveillance chief in Lebanon, General Rustom Ghazale, Syria’s former intelligence chief in Beirut, General Mohammed Khallouf, and Syrian General Jamii Jamii, were questioned by correspondence.

Mehlis has already questioned Mustafa Hamdan, head of Lebanon’s presidential guard and the highest-profile pro-Syrian official to keep his job in the political turmoil that followed Hariri’s murder.

France’s Le Figaro newspaper quoted Mehlis last month as saying the investigators suspected Hamdan of ordering the crime scene to be cleared. But a U.N. official later denied Mehlis had implicated anyone.

Lebanese Prime Minister Fouad Siniora said after meeting Mehlis on Friday the investigation was advanced but could require more time.

The U.N. Security Council resolution authorising the probe gave Mehlis three months to complete it.

He is due to report to U.N. chief Kofi Annan on Aug 25, but the Security Council can then grant his investigation an extra three months if he needs more time.

Friji said Mehlis “may likely need to seek an extension for a few weeks only.”