On this day in 1971, a group of activists broke into an FBI field office in Media, Pennsylvania and stole about a thousand government documents with a mysterious notation, “COINTELPRO.” The public revelation of these documents ranks with the Pentagon Papers as one of the most significant exposés of government secrets in U.S. history.

The burglars, who have never been identified, entered the two-person office in Media as much of the nation was huddled around television sets to watch the Muhammad Ali-Joe Frazier fight. The activists did not fully understand the documents they found, but they quickly decided that the public had the right to see them.

About two weeks later, two prominent antiwar lawmakers and reporters at major newspapers received copies of the files in plain brown envelopes. Most of the recipients accepted the FBI’s judgment that the files were “secret”: the New York Times and Los Angeles Times did not write about the documents, and the legislators returned their sets to the FBI. But Washington Post editors believed that the public had the right to know about the spying. The Post broke the first COINTELPRO story on March 24, 1971, revealing how the bureau had used mail carriers and a campus switchboard operator to eavesdrop on a radical professor at Swarthmore College.

A Senate investigating committee headed by Frank Church of Idaho later revealed the vast reach of COINTELPRO, which was the acronym for the FBI’s counterintelligence program. FBI Director J. Edgar Hoover started the operation in 1956 in response to Supreme Court rulings that made it more difficult to prosecute Communists. Under COINTELPRO, the bureau recruited “informants” – a euphemism for “informer” – to infiltrate the dwindling ranks of the Communist Party, disrupt its plans, and discredit its members. COINTELPRO agents planted “snitch jackets,” or false letters identifying a target as an informer, wrote anonymous poison pen letters, and spread rumors about political apostasies and marital infidelities. In other words, the FBI did not just monitor these individuals, but tried to break up their marriages, “seed mistrust, sow misinformation,” and provoke them to commit crimes so that they could be arrested.

The FBI originally directed this program at American Communists, but it soon broadened its definition of communism. By 1960, when the Communist Party counted about five thousand members in the United States, the bureau maintained more than eighty times that number of files on “subversive” Americans at its headquarters, and FBI field offices around the country collected even more. One purpose of COINTELPRO, according to an official memo, was to “enhance the paranoia endemic in [dissident] circles” and convince activists that “there is an FBI agent behind every mailbox.” The agents believed that paranoid, divided dissident groups were easier to handle than purposeful, united dissident groups. In other words, the FBI conspired to create fear of conspiracy.

By the mid-1960s, COINTELPRO had expanded to spy on, infiltrate, and disrupt a wide variety of activist groups, including the antiwar movement, women’s liberation groups, civil rights organizations, and the black power movement. The FBI also targeted some “white hate” groups like the Ku Klux Klan, but most of its efforts went into disrupting the left. Most notoriously, FBI officials spied on Martin Luther King, Jr. At Hoover’s direction, agents wiretapped King’s phones, bugged his hotel rooms, and did everything they could to take him “off his pedestal and to reduce him completely in influence,” as one FBI memo put it. The FBI peddled evidence of King’s extramarital affairs to public officials and journalists. Just before King was to accept the Nobel Peace Prize in 1964, the assistant FBI director sent the new laureate his own copy of the evidence. King received a composite tape in the mail that included audio recordings of his alleged trysts. A letter sent with the tape concluded with this threat: “King, there is only one thing left for you to do. You know what it is. …You are done. There is but one way out for you.” The FBI, in other words, tried to persuade the internationally recognized leader of the American civil rights movement to kill himself.

The Church Committee denounced COINTELPRO as a “sophisticated vigilante operation aimed squarely at preventing the exercise of First Amendment rights of speech and association.” With all due respect to the committee, FBI agents are, by definition, not vigilantes; they’re agents of the state. In this case, they were state agents of repression. It took vigilantes who fought for the right to know – those burglars in Media – to bring this secret government program to public attention.