In a collision of 21st-century science and decades-old conspiracy theories, a research team that includes a former top FBI scientist is challenging the bullet analysis used by the government to conclude that Lee Harvey Oswald alone shot the two bullets that struck and killed President John F. Kennedy in 1963.

The "evidence used to rule out a second assassin is fundamentally flawed," concludes a new article in the Annals of Applied Statistics written by former FBI lab metallurgist William A. Tobin and Texas A&M University researchers Cliff Spiegelman and William D. James.

The researchers' re-analysis involved new statistical calculations and a modern chemical analysis of bullets from the same batch Oswald is purported to have used. They reached no conclusion about whether more than one gunman was involved, but urged that authorities conduct a new and complete forensic re-analysis of the five bullet fragments left from the assassination in Dallas.

"Given the significance and impact of the JFK assassination, it is scientifically desirable for the evidentiary fragments to be re-analyzed," the researchers said.

Tobin was the FBI lab's chief metallurgy expert for more than two decades. He analyzed metal evidence in major cases that included the 1995 Oklahoma City bombing and the 1996 explosion of TWA Flight 800 off Long Island.

After retiring, he attracted national attention by questioning the FBI science used in prosecutions for decades to match bullets to crime suspects through their lead content. The questions he and others raised prompted a National Academy of Sciences review that in 2003 concluded that the FBI's bullet lead analysis was flawed. The FBI agreed and generally ended the use of that type of analysis.

Using new guidelines set forth by the National Academy of Sciences for proper bullet analysis, Tobin and his colleagues at Texas A&M re-analyzed the bullet evidence provided to the 1976 House Select Committee on Assassinations to support the conclusion that only one shooter, Oswald, fired the shots that killed Kennedy.

Now-deceased University of California at Irvine chemist Vincent P. Guinn. told the committee that he used bullet lead analysis to conclude that the five bullet fragments recovered from the Kennedy assassination scene came from just two bullets, which were traced to the same batch of bullets Oswald owned. Guinn's conclusions were consistent with the 1960s Warren Commission Report that found Oswald had acted alone. The House assassinations committee, however, concluded that Oswald probably was part of a conspiracy and that it was possible a second shooter fired one shot that missed the president.

Tobin, Spiegelman and James said they bought the same brand and lot of bullets used by Oswald and analyzed their lead using the new standards. The bullets from that batch are still on the market as collectors' items.

They found that the scientific and statistical assumptions Guinn used -- and the government accepted at the time -- to conclude that the fragments came from just two bullets fired from Oswald's gun were wrong.

"This finding means that the bullet fragments from the assassination that match could have come from three or more separate bullets," the researchers said. "If the assassination fragments are derived from three or more separate bullets, then a second assassin is likely," the researchers said. If the five fragments came from three or more bullets, that would mean a second gunman's bullet would have had to strike the president, the researchers explained.