WASHINGTON (Reuters) - Bullet analysis used to justify the lone assassin theory behind President John F. Kennedy's assassination is based on flawed evidence, according to a team of researchers including a former top FBI scientist.

Writing in the Annals of Applied Statistics, the researchers urged a reexamination of bullet fragments from the 1963 shooting in Dallas to confirm the number of bullets that struck Kennedy.

Official investigations during the 1960s concluded that Kennedy was hit by two bullets fired by Lee Harvey Oswald.

But the researchers, including former FBI lab metallurgist William Tobin, said new chemical and statistical analyses of bullets from the same batch used by Oswald suggest that more than two bullets could have struck the president.

"Evidence used to rule out a second assassin is fundamentally flawed," the researchers said in their article. "If the assassination (bullet) fragments are derived from three or more separate bullets, then a second assassin is likely."

The Kennedy assassination set off a whirlwind of theories about who killed the 46-year-old president.

The President's Commission on the Assassination of President Kennedy, known unofficially as the Warren Commission, concluded in 1964 that Lee Harvey Oswald, acting alone, fired three shots, one of which missed the president's car. There have been many challenges to its conclusions over the years.

The House of Representatives Select Committee on Assassinations concluded that Oswald was probably part of a conspiracy that could have included a second gunman who fired but missed Kennedy.

The panel's supporting evidence was a bullet analysis that said fragments collected from the site were too similar to be from more than two slugs.

But the latest report found that many bullets from the same batch used by Oswald had a similar composition.

"Further, we found that one of the thirty bullets analyzed in our study also compositionally matched one of the fragments from the assassination," the article said. "This finding means that the bullet fragments from the assassination that match could have come from three or more separate bullets."