Kopp's 1987 Chevy Cavalier
Anyone with any information on Dr. Slepian's killing is asked to contact: Amherst, New York Police Dept: 716-689-1390 New York State Police: 716-343-2200 FBI (Buffalo, New York office): 716-856-7800

AMHERST, New York (CNN) -- A car belonging to James Kopp, a man sought for questioning in the sniper killing of a New York abortion provider, has been located in New Jersey, authorities said Wednesday.

Dr. Barnett Slepian was killed October 23 by a shot fired from a high-powered rifle through the kitchen window of his home in suburban Buffalo, New York.

The car,a 1987 black Chevrolet Cavalier, had been spotted in Slepian's neighborhood in the weeks before the 52-year-old obstetrician-gynecologist was killed. It was discovered by a police officer on December 18 in a parking lot at Newark International Airport where it had been parked for an "unusually long time," said one investigator.

Kopp
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CNN's Pierre Thomas reports
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The Vermont license plate had been removed and replaced with a tag from another state, authorities said. They declined to say what, if anything, was found in the vehicle.

Kopp, 44, a radical abortion protester, is not being called a suspect, but authorities have obtained a material-witness warrant for his arrest and are searching for him throughout the United States, Canada and Mexico.

The U.S. government is offering a $500,000 reward in the case.