An audio recording has been played of an infamous 1970 Vietnam War protest at a US university in which the National Guard killed four students.
Mr Canfora found the tape and transferred it to CD
It was aired by Alan Canfora, one of several other students injured at the Kent State University rally, who said there should be a new investigation.
On the tape an officer appears to issue the command: "Get set! Point! Fire!"
For many activists the killings symbolised an end to the 1960s hippie era and the violation of civil rights.
Mr Canfora, 58, told the Associated Press he had been researching the shootings and came across the recording six months ago.
He said it had been given to Yale University by a lawyer who had represented students in their legal actions over the incident.
The tape was said to have been recorded by student Terry Strubbe via a microphone placed on the windowsill of his dormitory.
Mr Canfora played the original recording, plus an amplified one, copied on to CD, at Kent State campus on Tuesday.
Reporters said it was fuzzy but seemed to carry the command "Right here! Get set! Point! Fire!". They said the word "point" and gun shots were clear but it was not known who gave the order.
It is known that guardsmen fired 67 bullets from M-1 rifles into the crowd over a period of 13 seconds.
Four people died, including two who were not protesters but were walking from one class to another. Nine people were hurt.
Nixon defended attacks into Cambodia that led to protests
Mr Canfora said: "We're hoping for new investigations and new truths. We need truth, we need healing."
He was confident voice analysis would determine who gave the order to fire.
Whether there was an order to fire has always been contentious. The FBI at the time said it could come to no conclusion.
On Tuesday, a guardsman at the shootings, Larry Shafer, told AP: "I never heard any command to fire... That's not to say there may not have been, but with all the racket and noise, I don't know how anyone could have heard anything that day."
After the killings, activists criticised the thoroughness of the investigation.
Eight guardsmen were finally brought to trial on federal civil rights charges but each was acquitted.
The Kent State demonstration was one of many in protest at US attacks into Cambodia, which opponents of the war saw as an escalation but which President Richard Nixon defended.
There had been a clash between students throwing stones and guardsmen firing tear gas but why the shooting began is unknown.
Some have conjectured one shot was fired and other guardsmen followed suit in a chain reaction.