NEW YORK (Reuters) - A federal lawsuit filed on Wednesday revives lingering questions about the bloodshed at Kent State University, a traumatic event in U.S. history that still holds sway over many who lived through it 46 years ago.

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The lawsuit, filed by Buffalo attorney Michael Kuzma, asks the Federal Bureau of Investigation to release all records about a mysterious figure in the events of May 4, 1970, when Ohio National Guard troops shot dead four unarmed people and wounded nine others during a campus protest against the Vietnam War.

The deaths inflamed emotions in a country already polarized over the war and left an image seared in the national memory: John Paul Filo’s Pulitzer Prize-winning photograph of Mary Ann Vecchio grieving over the body of victim Jeffrey Miller.

The investigation left many who followed the case unsatisfied, and unanswered questions surround a man named Terry Norman.

Norman, 67, was a student and FBI informant. He carried a .38 handgun and a camera that day and his precise role has been disputed ever since.

Some experts speculate he fired the shots that provoked the National Guard’s volley, although he told investigators he never shot his gun. Others say he may have fired rounds 70 seconds earlier that had no bearing on the National Guard.

Norman has kept quiet, declining media inquiries over the years. Reuters could not reach him at his last listed telephone number and email address.

“Terry Norman is in a sense the second gunman on the grassy knoll ... He’s been kind of a handy bogeyman or a red herring,” said Howard Means, author of the recently released “67 Shots: Kent State and the End of American Innocence.”

The FBI cited privacy protections for not releasing documents about Norman or even confirming they exist, saying in a statement there would first need to be “an overriding public interest,” among other considerations.

Means and William A. Gordon, author of the 1990 book “Four Dead in Ohio,” said they believe the FBI has released all relevant documents but they support another effort to force the agency to check again.

Kuzma filed a Freedom of Information Act request last year for all FBI records on Norman. He was denied, Kuzma said in his lawsuit, which names the Justice Department, the FBI’s parent agency, as defendant.

“My hope is that we’re able to pry loose some documents that haven’t previously been released,” said Kuzma, a free speech advocate who has used the act to secure government documents on other topics.