A 40-year-old audio recording of the moments just before Ohio National Guardsmen opened fire on antiwar protesters at

will finally be professionally analyzed to try to determine if -- as some claim -- an order to shoot is audible.

The recording was made on May 4, 1970, by Terry Strubbe, a KSU communications student who set the microphone of his reel-to-reel tape recorder on his dorm room windowsill, turned on the machine, and went outside to watch the unfolding protest.

The chilling 30-minute tape is the only known audio that captured sounds before the shootings, the 13-second fusillade and its chaotic aftermath. Four students were killed and nine wounded in the incident, which spawned numerous inquiries and crystallized American sentiment about the unpopular Vietnam War.

The question of why 28 Guardsmen pivoted, raised their rifles, pistols and shotguns and fired 67 times at the students is the central mystery from that bloody Monday long ago.

Some of the soldiers, who had been pelted by rocks before the shooting, said students were advancing on them and they feared for their lives, although the presidential commission that investigated the event found that the leading edge of the crowd was at least 60 feet away.

A few students and Guardsmen claimed they heard something that sounded like an order to fire, but most of the men who acknowledged using their weapons later testified they acted spontaneously. The presidential commission, while acknowledging that the facts were in "bitter dispute," reported that "the weight of evidence" indicated no such firing command was given, either verbally or by gesture.

A Massachusetts acoustics firm scrutinized Strubbe's tape in 1974 at the request of the Justice Department, which was prosecuting eight of the Ohio Guardsmen. But the firm's analysis was limited to the gunshots; it did not address whether the tape contained an order to fire.

A federal judge dismissed the charges. A subsequent civil lawsuit filed by the shooting victims and their families against the Guardsmen eventually was settled with no admission of guilt.

Acoustic technology has advanced considerably in the 36 years since the recording was previously examined.

During that time, Strubbe has kept the tape in a climate-controlled bank vault. "He knew it was an historical document and was important," said his friend Joe Bendo. "He could have sold it. He wanted to make sure it was used in a factual and appropriate way."

Bendo, an Akron psychologist and sometime-TV commercial and music producer, is collaborating with Strubbe to have the tape analyzed.

They have commissioned a Los Angeles film archivist to convert its contents to a digital format, and to reduce background noise. Acoustic forensics experts then will try to determine if a firing order is audible. Bendo said he and Strubbe hope to produce a documentary later this year that will reveal the findings.

Alan Canfora, one of the nine KSU students wounded in the shootings, discovered a copy of the Strubbe tape several years ago while researching a book, and is convinced it contains a firing order.

Canfora, who runs the nonprofit Kent May 4 Center, located the cassette copy among materials that one of the lawyers in the shooting victims' civil lawsuit donated to Yale University.

In 2007 Canfora played excerpts of the Yale recording at a news conference. He said the command, "Right here, get set, point, fire!" was audible just before the shooting began. Some skeptics have said the phrase Canfora claims to hear isn't consistent with a military-style command.

The recording also has snatches of protest chants, a police announcement ordering students to leave the area "for your own safety," the sounds of tear gas grenades being launched, the pealing of the KSU victory bell as the protest escalated and, after the gunfire, an ambulance siren's wail.

On Friday, Canfora will hold another news conference to ask that state and federal officials reopen the Kent State investigation, based on the new evidence he says the tape contains. Scott Wilson, a spokesman for the FBI's Cleveland office, said, "Certainly if there are new findings or new information, we would review that."

Canfora said he doesn't want a punitive inquiry against the soldiers, but one that sets the historical record straight. "Personally, I will strongly oppose any attempts to prosecute these people," he said in an interview with The Plain Dealer this week. "I no longer feel a sense of anger and rage toward the Guardsmen as I did when I was younger. We do not seek retribution. We only seek the truth."

Sanford Rosen, one of the attorneys who represented Canfora and others in the civil lawsuits, said if the tape confirms a firing command was issued, it's not clear what the legal implications would be.

"I would think the statutes of limitations pretty well would have run [out] in terms of legal claims that could be brought," Rosen said. "But who knows? Strange things do happen."

The tape's historical significance is greater than its legal ramifications, the attorney said.

"Although many events have overtaken the Kent State shootings, it still looms very large in the consciousness of the American people," he said. If a Guard commander gave an order to fire, "that makes a huge difference to how we as a society will look back on the events."