The suspect, James Kopp, apparently escaped a federal dragnet in New Jersey several weeks ago with the aid of fellow anti-abortionists who drove him across the border, sources told CBS News.
Sources said Kopp, an anti-abortion activist, has apparently been on the move since arriving in Mexico and that investigators have not yet pinpointed his location. Mexican federal police are now actively assisting in the search.
Kopp's 1987 Chevrolet Cavalier was discovered Friday at Newark International Airport, the FBI and police in this Buffalo suburb said Wednesday.
Authorities have said the car was spotted in Amherst in the weeks before obstetrician-gynecologist Dr. Barnett Slepian, 52, was killed in his kitchen Oct. 23 by a rifle shot fired from outside.
Dr. Barnett Slepian
The attack was similar to four others against abortion providers that have taken place in Canada and western New York.
Kopp, 44, is has been sought for questioning in the killing since being named as a material witness Nov. 4. Officials have said the native of St. Albans, Vt., is not a suspect.
Investigators believe the killing is linked to four earlier nonfatal shootings that targeted doctors who provided abortion services in Canada and upstate New York.
The car had been in the long-term lot about two weeks, said Greg Trevor, a spokesman for the Port Authority of New York and New Jersey.
The car had a New Jersey license plate that didn't belong on it. FBI Agent Monica Patton in Newark said she couldn't say whether or not the plate had been stolen.
Police would not say what was found inside the car or whether they believed Kopp had left the country.
Kopp is believed to have been working on a construction job in the metropolitan New York area two days after the shooting, FBI Agent Bernard Tolbert said at a news conference.