Mr. Hasan was studying for his exam

"A few years after Hariri's death in a massive car bomb in 2005, there were reports that Mr Hassan was suspected of involvement.  As Mr Hariri's chief of protocol, he would normally have been in the convoy that was hit by the blast but had taken the day off to take a university exam.  UN investigators were reported in 2010 to have found this alibi unlikely, but Mr Hariri's son Saad said he had always had full confidence in Mr Hassan.   The following year, Mr Hassan was not among those indicted by the UN-backed tribunal. Instead, prosecutors sought the arrest of four members of the pro-Syrian Shia Islamist movement, Hezbollah." (thanks Samer)