On March 8, 1971, a group of burglars broke into an FBI office outside Philadelphia and stole thousands of files exposing a covert campaign to spy on antiwar activists and other dissidents the law enforcement agency suspected of subversive behavior.

After the embarrassing break-in, FBI Director J. Edgar Hoover dispatched hundreds of agents to try to catch the perpetrators. Despite all its resources, the agency couldn’t find them.

While the case floundered, the incriminating documents surfaced in newspapers nationwide, disclosing a dark period in the FBI’s history and humbling the powerful Hoover.

The stolen material was the first glimpse of an agency operation called COINTELPRO — an illegal and secret surveillance program targeting Americans, and one that used dirty tricks and smear tactics.

On Tuesday, some of the burglars involved in the heist were identified in a new book by former Washington Post reporter Betty Medsger, who first wrote about the stolen documents in 1971.

Bonnie Raines — with her husband, John — holds a 1971 FBI drawing of her that was done after the couple and others broke into and took files from an FBI office in Media, Pa., in 1971. (MICHAEL S. WIRTZ/Associated Press)

Because a five-year statute of limitations has long since passed, those responsible can’t be prosecuted.

The revelation rekindles interest in a period in the country’s history when Americans fiercely debated government intrusions in the name of rooting out supposed threats. The book, “The Burglary,” comes at time when a similar, contentious debate is taking place in the United States about privacy in the wake of former contractor Edward Snowden’s disclosures about National Security Agency surveillance programs.

“We hope that by coming forward today we can remind Americans of the abuses that unchecked police power can lead to, and contribute in some small way to the ongoing public debate over these recurring issues,” said Keith Forsyth, 63, an electrical engineer, who along with seven others planned and executed the burglary. They suspected that the FBI was involved in questionable tactics, but needed evidence.

The ringleader was William Davidon, a physics professor at Haverford College in Pennsylvania who turned to some trusted associates for help . The group gave itself a name: the Citizens’ Commission to Investigate the FBI.

And so it did.

Davidon and his band studied the agency’s office in Media, Pa. Much of the planning took place at the house of Bonnie and John Raines in North Philadelphia. Bonnie Raines, a mother of three young children back then, dressed as a college student and visited the office under the guise that she wanted to learn more about opportunities for women in the FBI, she said in a conference call with reporters.

Raines said they were on a vital mission. “We were concerned about the protection of our children’s rights — of people’s rights,” she said.

She saw that FBI file cabinets were not locked and that the office had no alarm. They could pull this off. With a crowbar, and on the night of a Muhammad Ali-Joe Frazier prize fight, they managed to get inside and steal thousands of documents.

The FBI played down the break-in at first. Two weeks later, Medsger, who had covered antiwar protests for the Evening Bulletin in Philadelphia but had since moved to The Washington Post, received a large envelope with 14 FBI files.

On March 25, she published her first blockbuster article on the front page of The Post.

“The documents describe FBI surveillance of campus and black activists organizations at one college campus, utilizing local police, letter carriers and campus employees,” she wrote.

Reporters eventually figured out that these documents were the underpinning of COINTELPRO.

One document described how the agency wanted to “enhance paranoia and let the people know the FBI was behind every mailbox,” Medsger said in an interview Tuesday.

Later, the full extent of the FBI’s activities and abuses would come out in congressional hearings and lead to major changes at the agency.

“A number of events during that era, including the burglary, contributed to changes in how the FBI identified and addressed domestic security threats, leading to reform of the FBI’s intelligence policies and practices, including the creation of investigative guidelines by the Department of Justice,” FBI spokesman Michael Kortan said in a statement.

The FBI closed the burglary case in 1976. Medsger said the agency had narrowed the list of hundreds of potential suspects to seven, but only one of them was from the Citizens’ Commission to Investigate the FBI.

John Raines, who had been a Freedom Rider in 1961, knew the agency would begin a massive dragnet to find them. He said two FBI agents came to his house in May 1971 to question him. When one asked if he had anything to do with the break-in, Raines made sure not to lie to the agent, which is a felony. He feigned outrage and refused to answer. The agents left.

Medsger didn’t forget the story or the dissidents she came to know through her reporting, including the Raineses. As the years passed, she stayed in occasional contact with them. In 1988, over dinner in Philadelphia, the couple disclosed their involvement to Medsger. “My mouth fell open,” she said.

In the conference call with reporters, John Raines summed up the past 43 years.

“Hoover lost. Freedom won.”