A young off-duty city EMT who had just started his career with the FDNY took his own life with a gun that belonged to his retired NYPD cop father, authorities and sources said Saturday.

John Mondello, 23, was found dead by the shoreline off Shore Blvd. in Astoria, just a few paces from Astoria Park, about 6:45 p.m. Friday. He died from an apparent self-inflicted gunshot wound to the head, police said.