James Kopp was one of the FBI's most wanted fugitives.
Erie County District Attorney Frank J. Clark said FBI officials informed him of the arrest Thursday morning but he had no other details.
``My first (reaction) was relief, and I guess the second was vindication for all those who said we'd never get him,'' Clark said. ``I never for a moment thought that he would not be captured. To me, it was a question of when.''
``I felt greatly relieved because I think this area cries for justice,'' Clark added. ``And, quite frankly, I let out a little 'whoopee,' too.''
The 52-year-old Slepian, just back from Sabbath eve services at his synagogue, was heating soup in his suburban Amherst home in October 1998 when he was gunned down with a single shot through a window.
Kopp, of St. Albans, Vermont, became the subject of an international manhunt the following month.
Nicknamed the ``Atomic Dog'' in anti-abortion circles, Kopp had been arrested in several states since 1990 for protesting abortion. His car was spotted in Slepian's neighborhood in the weeks before the shooting, and was found abandoned at Newark (New Jersey) International Airport in December 1998, authorities said.
Both charges carry a penalty of up to life in prison. The federal charge also carries a fine of up to $250,000.
Investigators said at the time that the discovery of a scope-equipped rifle buried near the Slepian home a few months after the shooting represented a major breakthrough. Slepian was shot with a rifle.
Kopp also had been linked, through DNA testing, to a strand of hair found near where the sniper fired, law enforcement sources have said.
Canadian authorities are eager to investigate Kopp because his movements in Canada coincide with the shooting of a Canadian abortion provider.
Offers of rewards for his arrest totaled $1 million.
The United States has an extradition agreement with France so it is expected Kopp will be returned to the United States to stand trial.