A plainclothes Nassau County cop responding to a 911 call of masked-man armed to the teeth with “Rambo-style” knives was shot dead last night by an MTA officer in a tragic case of mistaken identity, police sources told The Post.

The disastrous chain of events began at about 7:45 p.m. when a 27-year-old man — dressed in Army fatigues and a white hockey mask much like the knife-wielding maniac Jason from the “Friday the 13th” slasher flicks — began running around the streets of Massapequa Park, officials and witnesses said.

A frightened Theresa Kelly, who lives around the block on Front Street, said the loon pounced on her car while she was trying to back out of her driveway.

“I heard a loud thud against my window,” Kelly, 71, said. “I looked up and there was a man there wielding a knife and hitting my window.

“He was smirking and he was laughing. I screamed and he started to walk out of the driveway.”

The wacko ran back into his home on Fourth Avenue and barricaded himself in his bedroom, authorities said. Meanwhile, cops rushed the rest of the house’s occupants to safety.

“The guy’s father came out and said, ‘Don’t shoot my son, don’t shoot my son,” Kelly said.

But moments later the man — armed with “Rambo-style” knives — lunged at the officers and was killed when cops opened fire, authorities said.

It’s not exactly clear what happened next, but an MTA cop who responded from the nearby LIRR station then shot the plainclothes cop in the face, police sources said.

The wounded officer, who was assigned to the elite Bureau of Special Operations, was rushed to Nassau University Medical Center where he died.

He’s the second Nassau cop killed in the line of duty in recent months. Michael Califano, 44, was killed when a trucker fell asleep and slammed into his parked patrol car.

Nassau County police said the shooting was accidental but did not say who shot the officer.

The MTA declined comment.

The names of the cops and the suspect were not released.

Additional reporting by Rebecca Harshbarger