The LAPD does not believe ex-NFL player Jonathan Martin's gun photo on Instagram was a harmless social media prank -- they're treating it as a "serious threat" ... this according to sources directly involved in the investigation.
Multiple sources tell TMZ Sports ... Martin recently purchased 2 firearms -- and the gun pictured appears to match up with one of the guns he recently bought.
We're told police are working with all 4 individuals tagged in Martin's post to make sure they're safe.
As we previously reported ... Martin was detained by police Friday after word spread of his threatening post. Harvard-Westlake School in L.A. -- which Martin also mentioned in his IG pic -- cancelled classes Friday out of an abundance of caution.
It now seems the move was justified.