Just last week, we marked the 40th anniversary of the shootings at Kent State University that resulted in the death of four students. Now, there’s new evidence that sheds light on the events of that day:

The Ohio National Guardsmen who fired on students and antiwar protesters at Kent State University on May 4, 1970 were given an order to prepare to shoot, according to a new analysis of a 40-year-old audio tape of the event.

“Guard!” says a male voice on the recording, which two forensic audio experts enhanced and evaluated at the request of The Plain Dealer. Several seconds pass. Then, “All right, prepare to fire!”

“Get down!” someone shouts urgently, presumably in the crowd. Finally, “Guard! . . . ” followed two seconds later by a long, booming volley of gunshots. The entire spoken sequence lasts 17 seconds.

The previously undetected command could begin to explain the central mystery of the Kent State tragedy – why 28 Guardsmen pivoted in unison atop Blanket Hill, raised their rifles and pistols and fired 67 times, killing four students and wounding nine others in an act that galvanized sentiment against the Vietnam War.

The order indicates that the gunshots were not spontaneous, or in response to sniper fire, as some have suggested over the years.

“I think this is a major development,” said Alan Canfora, one of the wounded, who located a copy of the tape in a library archive in 2007 and has urged that it be professionally reviewed. “There’s been a grave injustice for 40 years because we lacked sufficient evidence to prove what we’ve known all along – that the Ohio National Guard was commanded to kill at Kent State on May 4, 1970.”

“How do you spell bombshell?” said Barry Levine, whose girlfriend Allison Krause was mortally wounded as he tried to pull her behind cover. “That is obviously very significant. The photographic evidence and eyewitness accounts of what took place seemed to suggest everything happened in those last seconds in a coordinated way. This would be the icing on the cake, so to speak.”