One of the many famous images from the Kent State shootings, which left four students dead.

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n a bombshell announcement that could rewrite the history of a defining event of the Vietnam War era, analysis of a 40-year-old audio recording suggests that Ohio National Guardsmen were given direct orders to fire at anti-war protesters at Kent State University. The 28 Guardsmen fired 67 shots, killing four students and wounding nine others. Despite several investigations and a lawsuit, there has never been a firm understanding of why the Guardsmen began shooting that day. Here's what's now known:

Where did the audio recording come from?
It was made by a student who put a microphone on his dorm-room windowsill to capture sounds from the protest. Forensics experts Stuart Allen and Tom Owen — at the request of the Cleveland Plain Dealer newspaper — removed extraneous noises from the recording, such as wind blowing across the mic, and were able to decipher formerly inaudible sounds. (Listen to the recording)

What did the analysis find?
After the estimated 2,000 protesters had been ordered to disperse, someone — presumably a National Guard commanding officer — shouted "All right, prepare to fire!" Other voices say, "Get down!" Then a voice says, "Guard!..." and two seconds later the shooting begins. The fusillade lasts 13 seconds.

Has anyone claimed that Guard troops were not ordered to open fire?
Yes. Many troops who fired that day have said that no order was given. A presidential commission also found "that 'the weight of evidence' indicated no such firing command was given, either verbally or by gesture." Many observers have concluded that the shooting was spontaneous, a response to sniper fire, or that one soldier panicked, inciting others to follow suit. Many of the protesters, however, have maintained all along that the soldiers were following orders.

Does this put the mystery to rest?
Not completely. The commanding officers who were at Kent State that day have all died — and without a voice sample for comparison, Allen and Owen can only speculate about who said, "Prepare to fire!" Also, a guardsman who was present on the of the protest tells AP that what's heard on the tape "does not seem consistent with how military orders are given."

Will the tape have any legal implications?
Probably not. Eight Guardsmen charged with civil rights violations were acquitted in 1974, and survivors and families of those who died settled a lawsuit in 1974.

Sources: Crooks and Liars, Associated Press, Cleveland Plain-Dealer