In an apparent case of fatal friendly fire, a uniformed police officer shot a plainclothes Nassau cop who had just arrived to an earlier call for backup at a home in Massapequa Park, authorities said.

The Saturday night shooting came moments after the uniformed police had already shot and killed a suspect in the home who had lunged at them with knives, a police source said.

The slain officer -- a member of the department's special operations bureau -- was pronounced dead at 9:19 p.m. at Nassau University Medical Center in East Meadow, said Det. Lt. Kevin Smith, the county police department's chief spokesman. The plainclothes officer was armed, said Smith, who declined to say if the cop's weapon was drawn at the time he was shot.

"It appears that it might have been an accidental shooting," Smith said. "He died at NUMC. His family was with him."

Police did not release the name of the officer -- the second Nassau cop to sustain fatal injuries in the line of duty in less than six weeks.

The suspect in Saturday's shooting, who also was not identified, was fatally shot inside his home before the officer was shot outside, the police source said. Cops with flashlights were searching the front yard lawn on Fourth Avenue just before 5 a.m. Sunday. Plainclothes and uniform officers were seen going in an out of the house, and just after 7 a.m. a body was removed on a stretcher.

Police did not say how much time elapsed between the two shootings.

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Two officers from the Seaford-based Seventh Precinct and an MTA police officer, who had responded from a nearby Long Island Rail Road station, responded to the initial 911 call and were involved in the initial shooting, Smith said.

He did not elaborate on the role of each officer, adding only that those officers also were taken to NUMC for reasons he did not specify. Nassau County Executive Edward Mangano visited the hospital, Smith said.

Hospital spokeswoman Shelley Lotenberg said the bullet wounded the slain officer in the left and right side of his chest and caused fatal trauma to his heart and lungs.

The dead suspect, 27, had large knives strapped to his body, Smith said.

Police first responded to an 8:20 p.m. 911 call, Smith said.

Theresa Kelly, 71, who lives near the site of the shooting, said she heard -- after police had evacuated the home -- the suspect's father shout to police, "Please don't shoot my son! Please don't shoot my son!' "

Kelly had called 911 a short time earlier when a man -- later identified as the 27-year-old suspect -- jumped on her car hood as she was backing out of her driveway.

The man was wearing a white mask and dark clothing, she said.

Kelly said she screamed and heard him laugh.

"He got a good laugh out of scaring me," Kelly said. "I thought he wanted to get into my car, so I called 911."

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When the man ran away, she followed him in her car and told police where he was, Kelly said. She watched a police officer with a gun go into a house, which is nearby on Fourth Avenue.

"A minute later, I heard three shots," Kelly said.

Smith said the suspect, who shared the home with his parents, barricaded himself inside the house as police arrived and hid in a bedroom.

Police evacuated the home, and soon after, the suspect charged out of the bedroom holding "big Rambo-type knives," Smith said.

At least two of the officers fired shots, and the suspect died of his wounds, police said. His body remained in the house early Sunday as police awaited a search warrant, Smith said.

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Long Island Rail Road spokesman Sam Zambuto would say only that the MTA police, who patrol the railroad, had responded to a situation in the Massapequa area Saturday night.

Last month, Nassau police mourned the department's first line-of-duty death since 1993 after a highway patrolman was killed during a traffic stop on the Long Island Expressway.

The patrolman, Michael J. Califano, 44, of Wantagh, was killed when a trucker dozed off and slammed into the officer's stopped patrol car, police said.

John Kaley, 25, was arrested shortly after the crash on charges including criminally negligent homicide.

With Carrie Mason-Draffen, Víctor Manuel Ramos and Alfonso A. Castillo