The head of the UN inquiry into former Prime Minister Rafik Hariri's assassination, German prosecutor Detlev Mehlis, is expected to visit Syria Tuesday to interview security officials about the murder, said a Syrian source close to the case. "Mehlis is coming to Damascus tomorrow" to interview Syrian officials over Hariri's assassination in a massive Beirut bomb blast in February, he told AFP on Monday. His visit follows an initial trip to Damascus earlier this month during which he and the Foreign Ministry agreed on a format by which he would question the Syrian officials. The interviews will be a key test of the Syrian regime's readiness to cooperate following the arrest of four top allies in Lebanon. Mehlis is expected to question Interior Minister Ghazi Kenaan, a former military intelligence chief in Lebanon, as well as his successor Rustom Ghazaleh, who left along with Syrian troops in April, and two key aides in Beirut, Mohammad Khallouf and Jamaa Jamaa. According to As-Siyassah newspaper, Mehlis will also be questioning Maher Assad, the Syrian President's brother. Mehlis has said no Syrian suspect has been identified, but there are "more people involved" than the four Lebanese security chiefs arrested so far.