Police Investigating Alleged Masturbating San Diego Chargers Security Guard

The San Diego Chargers security guard who was allegedly caught masturbating on video while he stood on the field could lose more than his job ... because cops are now on the case.

Law enforcement in SD tells us they've been made aware of the incident and have opened up their own criminal investigation.

The video -- posted Monday -- shows a male guard allegedly pleasuring himself in front of a group of cheerleaders. Both the team and security company were quick to action ... investigating and firing the employee in a matter of hours.