Accused Craigslist killer Philip Markoff, charged with killing a New York masseuse in a Back Bay hotel room last year, was found dead in his jail cell yesterday morning, a plastic bag tied over his head in an apparent suicide, two law enforcement officials briefed on the investigation said yesterday. In April 2009, Markoff was examined by medical staff at the jail and put in a special cell under camera surveillance after a correction officer saw marks on his neck that suggested he had tried to use shoelaces to hang himself, the Globe reported at the time.