A Long Island cop who committed suicide this week was under investigation for having sex with his girlfriend while on duty — the second married Nassau County cop caught fooling around on the job in a month, sources said.

Officer Joseph Casazza, a 25-year veteran, was discovered by a jogger hanging from a tree in Mill Pond Park in Bellmore at 7:30 am. Monday.

“The charge being investigated is that he was visiting his girlfriend while on duty,’’ in uniform, in a squad car, said one source.

Officer Michael Tedesco, also married, was suspended Feb. 8 after he was caught visiting his girlfriend in his squad car while on duty.