Yesterday, Mr Canfora, who was among the nine students wounded on that day, demanded a new investigation into the shootings at Kent State in Ohio, saying it was time to settle conclusively what led the contingent of National Guard troops to open fire on unarmed student protesters.

"There has been a 37-year cover-up at Kent State. The commanding officers have long denied there was a verbal command to fire. They put the blame on the triggermen," Mr Canfora told the Guardian.

He said he wants the FBI to use new technology to analyse the recording. He also said he planned to post an audio clip of the recording on two websites.

Mr Canfora, who was 21 years old at the time of the shootings, was barely 60 metres away from the Guards when they opened fire. He was shot in the wrist.

"They stopped, turned, raised the weapons, began to shoot and continued to shoot for 13 seconds," he said. "It was like a firing squad."

His life was transformed by the events that day. One of his friends was among the dead, and he has devoted much of his time over the last 37 years trying to bring the Ohio National Guard and the federal authorities to account for the killings.

The Guard has always claimed that no order was given to open fire, and there is speculation that the students were cut down after one of the troops panicked, triggering a volley of gunfire.

Although eight guardsmen were indicted, no one was ever prosecuted, and the episode exposed the deep disdain of the Nixon administration for dissenters. The families of the 13 killed and wounded pursued a civil suit against the state governor and the National Guard, which was eventually settled out of court.

The materials from that civil suit were eventually stored in the archives at Yale University, where Mr Canfora recently rediscovered a 30-minute recording of the protest.

The recording was made by a fellow student, Terry Strubbe, who placed an old-fashioned reel-to-reel tape recorder on the window sill of his dorm room, which overlooked the protests. Mr Strubbe, who has declined to speak to reporters, still has the original recording in a bank safety deposit box.

However, a spokesman for Mr Strubbe, Joseph Bendo, told the Guardian yesterday he was unsure whether there were sounds of an order to open fire on the original recording.

"It was never heard on our version of the tape, but maybe nobody ever listened. It's unusual that nobody has heard it before in 37 years. Other people have heard this tape in the past, and maybe they weren't listening for it," he said.

But the power of America's memories of that day are undeniable. Nearly two generations after the shootings at Kent State, it now seems unthinkable that the National Guard could ever use live ammunition against students.

The events of that day were relived endlessly in shocking images of teenagers crouching over the corpses of their fellow students in the US heartland. They also led to protests which radiated across the country, shutting down hundreds of college campuses, and forcing Richard Nixon to decamp Washington for Camp David.

· To hear the recording go to Guardian.co.uk/world and follow the link