The chief UN investigator into the assassination of former Lebanese Prime Minister Rafik Hariri visited Syria under heavy guard Tuesday to interview President Bashar Assad, who is alleged to have threatened the Lebanese leader months before he was killed. Senior Lebanese security officials said that Chief UN investigator Serge Brammertz traveled by land to Damascus is a heavily guarded convoy of bulletproof vehicles. Syrian officials had no immediate comment on the visit and a spokesperson for the UN investigation in Beirut declined any comment. The convoy of Brammertz crossed back into Lebanon six hours later, the Lebanese officials said. No details were available about his talks in the Syrian capital. It will be the first time that Assad answers questions about Hariri's assassination from the UN commission that was appointed by the Security Council, which has accused Damascus of failing to cooperate fully with the probe into the February 2005 truck bombing that killed Hariri and 20 others on a Beirut street. Assad declined two previous requests for interviews filed last year by the commission, which is based in Beirut. Brammertz said in March that Assad accepted to meet with him. In a TV interview last month, Assad said that while he expected "a meeting, not an interrogation" with Brammertz, no question would be off-limits. "They can ask whatever they like," he said. Brammertz will also talk to Vice President Farouk al-Sharaa about Syria's alleged involvement in the assassination, the Lebanese security officials said, speaking on condition of anonymity because they were not authorized to speak to the media. Many Lebanese blame Syria for Hariri's assassination and for a series of mysterious bombings that have targeted Lebanese politicians and journalists opposed to Syria in the last 14 months. Syria denies involvement in all the attacks. According to testimony to the UN investigation by Hariri's political allies and family members as well as a former Assad vice president who defected, the Syrian leader threatened Hariri when they met in Damascus in 2004. Assad allegedly said he wanted the term of Lebanon's pro-Syrian president to be extended, a move that Hariri was known to oppose. Hariri, however, went along later with the extension of President Emile Lahoud's term. Assad has said in media interviews that Hariri was a friend of Syria and denied he threatened the late Lebanese politician. In a media interview last month, Assad suggested that Hariri may have lied about the threats against him to deflect criticism for siding with Syria in supporting the extension of Lahoud's term. "We heard later that he (Hariri) said that somebody from the Syrian Intelligence put a gun to his head, but Hariri himself told me that some officials in the West told him that they were angry with him because he stood by Syria. He told me that, but maybe he told them he did that for this reason," Assad told PBS television. "Actually, neither me nor anybody else in Syria threatened him," Assad added. The death of Hariri was turning point in Syria's relations with Lebanon. As he was seen as a quiet opponent of Syrian domination of Lebanon, Hariri's killing provoked mass demonstrations against Syria which, combined with international pressure, forced Damascus to pull its troops out of its western neighbor in April last year, ending a 29-year military presence. Four top Lebanese generals - key figures in Syria's domination of Lebanon - have been arrested and charged with involvement in Hariri's killing; top Syrian officials have been implicated, but not charged. Tuesday's visit is Brammertz's second to Syria since he took over the UN investigation from Detlev Mehlis in January. In February, Brammertz, a Belgian prosecutor, went to the Syrian capital and discussed Syrian cooperation with Foreign Minister Walid Moallem. Syria is under heavy international pressure to fully cooperate with the UN probe. Damascus has been harshly criticized by the United States and its Western allies - who threatened sanctions - for failing to fully cooperate in the probe. In his report to the UN Security Council last month, Brammertz said there are encouraging signals from Syria. The report noted that, after two high-level meetings, Syria agreed to a deal that will give the commission access to "individuals, sites and information."