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GILFORD, N.H. (AP) _ A law professor who served as counsel to the Warren Commission that investigated President Kennedy's assassination was killed in a plane crash during a training flight. His instructor also died.

Wesley J. Liebeler, 71, and flight instructor Alan Emerson, 58, were the only ones aboard the small plane when it crashed Wednesday in Lake Winnipesaukee.

It was not known who was at the controls, said Richard Eilinger of the Federal Aviation Administration.

Liebeler was a law professor at George Mason University in Virginia and one-time counsel to the Warren Commission. He was a licensed pilot getting additional instruction from Emerson.

The cause of the crash was under investigation.

In 1964, Liebeler interviewed key witnesses as assistant counsel to the President's Commission to Investigate the Assassination of President Kennedy. It was known as the Warren Commission because it was headed by Chief Justice Earl Warren. The commission concluded that Lee Harvey Oswald killed Kennedy and acted alone.

Liebeler ``was a fierce public defender of the Warren Commission,'' said David Lifton, author of ``Best Evidence,'' a book critical of the commission. ``But in private, he was open to criticism. He was not one of those slavish defenders.''

Jacob Cohen, a Brandeis University professor of American studies and expert on the Kennedy assassination, called Liebeler ``an independent voice who didn't go along right away with the consensus''

Liebeler was also a leading authority on antitrust law and professor emeritus at the University of California at Los Angeles, where he joined the faculty in 1965. Previously, Liebeler practiced law in New York City.

Survivors include his wife and two sons.