CLEVELAND (AP) — A new analysis of a 40-year-old audio recording reveals that someone ordered National Guard troops to prepare to fire on students during a deadly Vietnam War protest at Kent State University in 1970, two forensics experts said.

The recording was enhanced and evaluated by New Jersey-based audio experts Stuart Allen and Tom Owen at the request of The Plain Dealer newspaper. Both concluded that they hear someone shout, ‘‘Guard!’’ Seconds later, a voice yells, ‘‘All right, prepare to fire!’’

‘‘Get down!’’ someone shouts, presumably in the crowd. A voice then says, ‘‘Guard!…’’ followed two seconds later by a booming volley of gunshots.

Four Kent State students were killed and nine were wounded.

‘‘I think this is a major development,’’ said Alan Canfora, who was shot and wounded in the right wrist during the protest on May 4, 1970. Canfora, who has long believed that the troops were ordered to fire, located a copy of the tape in a library archive in 2007 and has urged that it be professionally reviewed.

The original reel-to-reel audio recording was made by Terry Strubbe, a student who placed a microphone in a window sill of his dormitory that overlooked the anti-war rally.

Allen, president and chief engineer of the Legal Services Group in Plainfield, N.J., removed extraneous noises — wind blowing across the microphone, for example — that obscured voices on the recording.

Without a voice sample for comparison, the new analysis can’t determine who might have issued such a command or why.

Most of the senior Ohio National Guard officers directly in charge of the troops have died.

Ronald Snyder, a former Guard captain who led a unit that was at the Kent State protest but was not involved in the shootings, said the prepare-to-fire phrasing does not seem consistent with how military orders are given.

The FBI investigated whether an order had been given to fire and said it could only speculate. One theory was that a guardsman panicked or fired intentionally at a student and others fired when they heard the shot.

In 1974, eight guardsmen tried on federal civil rights charges were acquitted by a U.S. judge. The surviving victims and families of the dead settled a civil lawsuit for $675,000 in 1979, agreeing to drop all future claims against the Guardsmen.

The significance of the new audio analysis may be more historical than legal, said Sanford Rosen, one of plaintiffs’ attorneys in the civil lawsuit.