A cleaned-up new copy of an audio tape of John F Kennedy's assassination may prove whether killer Lee Harvey Oswald acted alone.
JFK was shot in Dealey Plaza
The sound recording could strengthen or destroy conspiracy theories if it establishes - as some believe it will - how many shots were fired.
US scientists have started making the first digital copy of the original tape - now too fragile to be played.
JFK was shot dead in his car in Dallas, Texas on 22 November 1963.
The sound of the event was caught by the radio on a nearby police motorcycle, and captured on a plastic Dictaphone belt at police headquarters.
In 1979 the US Congress House Select Committee on Assassinations said four shots could be heard on the tape, and they appeared to come from two different locations.
The committee concluded that Oswald, who was arrested for the murder but shot dead before he could be tried, probably did not act alone.
However, the sound of shots cannot be distinguished as such by the ear and the committee's report has been described as "seriously flawed" by the US National Academy of Sciences.
The motorcycle is now thought to have been near the Trade centre where Kennedy was due to have lunch.
The sound recording had not been used by the original Warren Commission investigation, which concluded that Oswald was the lone gunman and fired three shots from the Texas Book Depository.
Despite its apparently crucial role in the case, the Dictaphone belt has not been played or copied since 1990, because of fears over its deteriorating state.
Did Oswald act alone?
Now scientists at the Lawrence Berkeley National Laboratory have begun digital scanning of the belt to make a modern copy.
Leslie Waffen, an archivist at the US National Archives, believes digital analysis could be used to remove such extraneous noise as static and distant voices to reveal how many of the sounds were gunshots.
"This is big," said Mr Waffen. "That's why we called the experts in. They came up with a recommendation to do this."
The work may take a year or so, but some are impatient to hear the results.
"People want to know," said Gary Mack, of the Sixth Floor Museum at Dealey Plaza, where Kennedy was shot.
"The Warren Commission said it was one guy. The House committee said it was Oswald and someone else. There hasn't been any resolution."