Gabby Petito's death has put her fiance, Brian Laundrie, in the crosshairs of law enforcement, but TMZ has learned prosecutors are not making a move until two primary loose ends are tied up.Law enforcement sources with direct knowledge tell TMZ Laundrie is their suspect, despite the fact he's being officially called a person of interest. RELATED: Expert defense attorney says she 'wouldn't be surprised if there's an arrest' of fiancéThat said, TMZ is told federal prosecutors are now in a waiting game, in large part because they don't know if Laundrie is still alive.Law enforcement sources tell TMZ, even if Gabby was murdered -- which seems to be the case since the Coroner has already ruled her death a homicide -- they would need to prove who.