A former New York City police officer identified Darrell Fuller Wednesday as the man he saw at the scene of separate murders -- of a Nassau police officer and another man -- on the Nassau-Queens border in 2012.

Officer Robert Mancini, who retired last year, pointed at Fuller, seated at the defense table in County Court in Mineola. "He's wearing a black jacket with a white shirt, Mancini, 45, said.

He said he was off duty when he followed Fuller, 34, of St. Albans, Queens, from the first moment he saw his vehicle being pursued by cops on the Cross Island Expressway, through both shooting scenes and through local streets before losing him.

Mancini testified he was not carrying a weapon and was driving from the Bronx to Queens on Oct. 23, 2012, when he saw an Emergency Service Unit vehicle from the Nassau Police Department following a damaged car heading south on the parkway.

"They were trying to pull him over or stop him," Mancini said.

He also turned on the video recorder on his cellphone and captured part of the ensuing chase. That recording was played for the jury.

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He followed the police van and the damaged car off the highway at Jamaica Avenue, then back onto the southbound entrance of the parkway, near a Mobil station. His cellphone stopped recording just before he got to the station, where Fuller allegedly shot and killed one of the pursuing officers, Arthur Lopez, 29, of Babylon Village.

Mancini said he saw Lopez staggering as Fuller jumped back into his car, a semi-automatic pistol in his hand.

"I made the decision to follow him," he said, referring to Fuller. "I thought he [Lopez] would be assisted by his partner and he would be OK."

Mancini said a short time later he passed Fuller's Honda sedan, apparently disabled, on the side of the road. He said he saw Fuller pulling a man out of a Toyota Camry parked nearby on the shoulder.

Fuller is charged with fatally shooting that man, Raymond Facey, 58, of Brooklyn and stealing his car. Authorities have said Facey had pulled over to make a phone call.

Mancini told the jury he called 911 at the next exit. When Fuller passed in his hijacked vehicle, the officer followed him off the highway at Hempstead Turnpike and onto local streets.

That 911 tape was also played for the jury, who heard Mancini tell a dispatcher, block by block, where Fuller was going.

"I don't know where he is," Mancini finally says at the intersection of 209th Street and Murdoch Avenue. Fuller was arrested several hours later.

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Defense attorney Kenneth St. Bernard had just begun his cross-examination of Mancini when court adjourned until Friday. Fuller faces life in prison if convicted of first-degree murder.