WASHINGTON — 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 acted alone in assassinating President Kennedy in 1963.

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

The researchers’ reanalysis involved new statistical calculations and a modern chemical analysis of the same batch of bullets 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 reanalysis of the five bullet fragments left from the assassination 44 years ago.

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

Mr. 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 explosion of TWA flight 800 over Long Island in 1996.

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 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, Mr. Tobin and his colleagues at Texas A&M reanalyzed the bullet evidence used by the 1976 House Select Committee on Assassinations, which concluded that only one shooter, Oswald, fired the shots that killed Kennedy in Dallas.

The committee’s finding was based in part on the research of a now deceased University of California-Irvine chemist, Vincent Guinn. 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.

To do their research, Messrs. 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 the fragments came from just two bullets fired from Oswald’s gun were wrong.

“Matches of bullets within the same box of bullets are shown to be much more likely than indicated in the House Select Committee on Assassinations’ testimony,” the researchers wrote. “This finding means that the bullet fragments from the assassination that match could have come from three or more separate bullets.

“If the assassination fragments are derived from three or more separate bullets, then a second assassin is likely, as the additional bullet would not be attributable to the main suspect, Mr. Oswald,” the researchers said.