On the eve of the 42nd anniversary of the shootings, four students wounded that day asked U.S. Attorney General Eric Holder to investigate digitally enhanced audio evidence they believe proves an officer ordered the guardsmen to fire on the unarmed students.

A command to fire has never been proven and guardsmen said they fired in self-defense. Criminal charges were brought against eight guardsmen, but a judge dismissed the case. Wounded students and families of those slain later received a total of $675,000 after civil lawsuits.

The Justice Department closed an inquiry of the 2010 tape analysis last month, finding the enhanced audio was inconclusive.

The Kent State survivors, including Dean Kahler who was paralyzed from the waist down, are now requesting the Justice Department to offer immunity to any surviving guardsmen in hopes they will come forward with information. They say if DOJ refuses, they will "appeal to the International Court of Justice, the U.N. Human Rights Council or the Inter-American Court of Human Rights."

Here is what the audio analysis from 2010 concluded:

"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 tape was discovered in the library in 2007. Here's the recording released that year.

Here's the 2010 digitally enhanced version.

For readers too young to remember the Kent State shootings, here's an account from the BBC series, War and Protest, the U.S. in Vietnam":

In April, President Nixon promised to withdraw another 150,000 American troops from Vietnam over the coming year, and the United States military suspended the use of Agent Orange.

On 30 April, 1970, President Nixon announced that United States combat troops and B-52 bombers would enter Cambodia to destroy North Vietnamese and Vietcong sanctuaries and supplies. Excerpts from the speech in which he made that announcement include the following comments:

Ten days ago in my report to the nation on Vietnam I announced the decision to withdraw an additional 150,000 Americans from Vietnam over the next year....

... And at that time I warned that if I included that if increased enemy activity in any of these areas endangered the lives of Americans remaining in Vietnam, I would not hesitate to take strong and effective measures to deal with that situation.

Despite that warning, North Vietnam has increased its military aggression in all these areas, and particularly in Cambodia....

... Cambodia, as a result of this, has sent out a call to the United States, to a number of other nations, for assistance. Because if this enemy effort succeeds, Cambodia would become a vast enemy staging area and a springboard for attacks on South Vietnam along 600 miles of frontier: a refuge where enemy troops could return from combat without fear of retaliation....

... Tonight, American and South Vietnamese units will attack the headquarters for the entire Communist military operation in South Vietnam. The key control centre has been occupied by the North Vietnamese and Vietcong for five years in blatant violation of Cambodia's neutrality..

In the days following the Presidential announcement, students on University campuses across the United States were protesting the US invasion of Cambodia. At Kent State University in Ohio, protesters threw rocks and broke some windows. Some students tried to burn the ROTC (Reserve Officer Training Corps) building.

On 3 May, 1970, Ohio Governor James Rhodes called in the National Guard.

The National Guard units that responded were poorly trained and had just completed riot duty elsewhere. The first day, there was some brutality; members of the National Guard bayoneted two men, one of whom was a disabled veteran, who had cursed or yelled at them from cars.

On 4 May, the National Guard marched down a hill, to a field in the middle of angry demonstrators, then back up again. Seconds before they would have passed around the corner of a large building, and out of sight of the crowd, some of the Guardsmen wheeled and fired directly into the students, hitting 13 and killing four of them. The firing lasted for 13 seconds. Guardsmen later admitted to firing at specific unarmed targets; one man shot a demonstrator who was giving him the finger. The unarmed students who were shot raged from 60 feet to 700 feet away from the Guardsmen.

The targets were not limited to protesting students. Two of the four who had been killed were simply on their way to class. Most of the Guardsmen later testified that they turned and fired because everyone else had. The question of who fired the first shot, or gave the order to fire, has never been answered. The Guardsmen were not in any immediate physical danger when they fired. The demonstrators were not following them and they were seconds away from being out of sight of the demonstration.

The four students killed by members of the Ohio National Guard were: Alison Krause, Jeffrey Miller, Sandra Scheuer and William Schroeder.

The Guardsmen were never prosecuted by the State of Ohio, for any crime. President Nixon announced any number of investigations, none of which reached any clear conclusions. White House tapes released later showed that Nixon thought demonstrators were 'bums'3, had asked the Secret Service to go beat them up, and apparently felt that the Kent State victims 'had it coming'.

I remember where I was on May 4. 1970. I had just returned home to New York from college in Ann Arbor, MI, to begin my summer job at a local record store. The news spread like wildfire, even without internet, email and cable tv. We wore black armbands at work the entire next week and the music we played in the store reflected our anger.

Four years later (38 years ago today) May 4, 1974, I was sworn in as a lawyer to the Colorado bar and began my career as a defender of constitutional rights and the accused. Without a doubt, the draft lottery, the Vietnam war, LBJ, Hubert Humphrey and Richard Nixon were factors in my choice, and I'm proud to say I've never once looked back to question it.

Here is an excellent and prescient reminder of why we should not forget Kent State, written in 2000 by Mark Weisbrot.

To forgive is a virtue, but forgetting is an indulgence we can ill afford. Our foreign policy establishment remains addicted to empire, and is possessed by a hubris that is arguably even greater than the one that got us into Vietnam. Until they learn the lessons that the anti-war movement tried to teach them, we can expect more Vietnams ahead of us.

Those of us who write about Kent State on its anniversary every year may just be repeating our own words. But there's nothing wrong with that. We don't want the message to die. As Hunter Thompson's wife, Anita, wrote here at TalkLeft on the eve of the 2008 Democratic convention in Denver, while listening to iconic rock music photographer Lynn Goldsmith and I recount our protesting experiences during the Vietnam war, Hunter expressed it this way:

There is probably some long-standing "rule" among writers, journalists, and other word-mongers that says: "When you start stealing from your own work, you're in bad trouble." And it may be true. I am growing extremely weary of writing constantly about politics. My brain has become a steam-vat; my body is turning to wax and bad flab; impotence looms; my fingernails are growing at a fantastic rate of speed - they are turning into claws...People are beginning to notice, I think, but f*ck them. I am beginning to notice some of their problems too...

-- Hunter S. Thompson, Fear and Loathing on the Campaign Trail '72.

Here is the Kent State University Archive collection of the May 4, 1970 shootings.

Memo to commenters: In writing about 2010 analysis of the tape, I noted that one of the two experts enhancing and analyzing the tape in 2010 was Tom Owen. I shouldn't have to say this, but I will: Please keep your comments in this thread to your thoughts about Kent State, war protests, U.S. war policy, and similar topics. Owen's analysis of the 911 tapes in Zimmerman is not the topic.