(CNN) -- The Justice Department has declined to reopen an investigation into the 1970 shootings at Kent State University that left four student protesters dead, after the agency found that enhanced audio recordings of the incident were inconclusive as to whether an order to fire was given.
The students had been protesting the Vietnam War and the U.S. invasion of Cambodia when Ohio National Guard members opened fire.
Nine others were wounded in the incident.
The digitally enhanced 29-minute audio clip, originally recorded on a reel-to-reel machine on May 4, 1970, from the window of a university dorm, captured the sounds of the shootings, according to the Justice Department.
The agency's decision came last week in response to a letter from former student Alan Canfora, 63, who was shot in the wrist during the incident and submitted the evidence to authorities in 2010.
He says the digital version was dubbed from the original copy and contained proof of an order to fire, followed by 13 seconds of gunfire.
CNN has listened to the recording and cannot confirm that account. But Canfora says an independent analysis by an audio professional on the same recording verifies a clear command to fire before the deadly gunshots.
Eight Guardsmen were charged in 1974 for their alleged roles in the shooting but were acquitted because a judge ruled that the government could not prove its case.
"It's always been the central mystery," Canfora said. "Was there or was there not an order to fire?"
A federal investigation found the audio quality to be poor and "shouting to be unintelligible," according to a letter from Assistant U.S. Attorney General Thomas Perez of the Justice Department's Civil Rights Division.
The letter says "no military-like voice commands to fire or otherwise were heard," adding that a statute of limitations bars prosecutors from reopening the case.
Canfora argued that he never wanted the case retried but is seeking a "grand pronouncement of truth for the sake of the historical record."
"Were looking for truth and healing," he added. "When you have a lingering injustice, there's no real feeling of closure."
Canfora said victims of the shooting plan to announce May 3, the day before the 42nd anniversary, a plan to move the case to an international court.