On March 8, 1971, a group of eight Vietnam War protestors broke into a Federal Bureau of Investigation field office in Media, Pennsylvania and stole hundreds of government documents. The burglars were never caught but several are now stepping forward and claiming responsibility for the break in. The stolen memos, reports and internal correspondence they found provided the first tangible evidence that J. Edgar Hoover’s FBI was systematically targeting and harassing hundreds of American citizens then known collectively as “the New Left.”

Despite the efforts of nearly 200 investigators, the F.B.I. couldn’t find the burglars. But 43 years later, a new book by a journalist who received and reported on the documents, identifies the Media burglars for the first time. Betty Medsger’s The Burglary chronicles the planning, execution, and consequences of the long-forgotten heist, carried out by a group that included college professors, graduate students, and a cab driver. Their story is also chronicled in 1971, a new documentary by Johanna Hamilton.

The early 1970s was a turbulent time. Factions of the anti-war movement were becoming more aggressive and Hoover’s F.B.I. was stepping up its efforts to stop activities it considered a threat to national security. The situation worsened in the spring, when Nixon announced the U.S. invasion of Cambodia. Four student protesters were shot and killed by National Guardsmen at Kent State University. Thousands of anti-war protestors were angry, including Keith Forsyth. He was single, in his early 20s, and driving a taxi in Philadelphia, which, at the time, was a hotbed of dissent.

“I was pretty vehement in my opposition to the war,” says Forsyth. “I felt like marching up and down the street with a sign wasn’t cutting it anymore.”

John and Bonnie Raines were married and raising a family in the early 1970s. John was a religion professor at Temple University and Bonnie was working at a day care center. They had three young children and lived in the Germantown section of Philadelphia. Together, they attended rallies and marches protesting the war.

Like many others, Forsyth and the Raines suspected the F.B.I. was conducting a covert campaign to undermine the efforts of the New Left; but they couldn’t prove it. An acquaintance, a physics professor at Haverford College named William Davidon, came up with the audacious plot to break into the Media, Pennsylvania F.B.I. office. Davidon, who died in November 2013, believed the small outpost might contain documents that would implicate the F.B.I. in illegal activities. He assembled the team, which included Forsyth and the Raines, and devised a plan to break in and retrieve them.

They spent months meeting in the Raines’ third floor attic going over details. They cased the office building, memorized the layout of neighborhood, devised a getaway plan, and finally, decided on a date: March 8, 1971, the night of “The Fight of the Century,” the first bout between Muhammad Ali and Joe Frazier.

Forsyth—who had studied lock-picking—was critical to the burglars’ entry. An “inside team,” dressed in business suits, filled several suitcases with documents and made their way to an empty Pennsylvania farmhouse an hour’s drive from Media.

“We were all in one room,” says John Raines, “and there were different tables and each of us was sorting files and all of a sudden, you’d hear one of us yell, “Oh, look, look! Look at this one!”

The burglars sorted through hundreds of documents and mailed the most revealing ones to journalists, including Medsger, who was then a reporter at The Washington Post. The Post ran a series of stories that reported how the F.B.I. was spying on political activists and actively trying to disrupt their activities.

As part of the cache of documents, Medsger received an F.B.I. routing slip with a mysterious word on it—COINTELPRO. Years later, NBC’s Carl Stern would report that stood for “counter intelligence program,” Hoover’s vast covert operation to infiltrate and disrupt groups he considered subversive and a threat to national security.

In response to the exposure of COINTELPRO and other revelations, the Senate formed the so-called Church Committee, and Congress later issued reforms to limit how the government could gather intelligence. One result was the Foreign Intelligence Surveillance Act, or FISA court, which required government intelligence agencies to seek warrants before spying on U.S. citizens.

After 9/11, more wide-ranging surveillance was allowed. Just how much the N.S.A.’s spying activities had grown was largely unknown until 2013, when contractor Edward Snowden leaked a secret court ruling ordering Verizon to hand over phone call data on millions of United States citizens.

“My response to Snowden,” says Forsyth, “was here we go again.”