During the summer of 1964 hundreds of student volunteers travelled to Mississippi as part of a project to help local African-Americans to register to vote. Within two weeks there were at least seven bombings of buildings associated with the project, four shootings, and beatings of civil rights workers.

On June 21, 1964, three of these volunteers — Andrew Goodman, Michael Schwerner and James Chaney — went to investigate the burning of a black church in Neshoba County. They never came back. Their bodies were found 44 days later. According to one witness, Chaney’s corpse looked as if he had been in an air crash. It later transpired that Chaney, Goodman and Schwerner had been arrested for speeding, beaten with chains in jail, and then turned over to the Ku Klux Klan by the local sheriff (himself a member of the Klan). The mob shot each man before dumping the bodies in a dam.

Because Goodman and Schwerner were white students from the north, their disappearance captured national and international attention. President Lyndon Johnson ordered the FBI director J. Edgar Hoover to get a grip on the state. Hoover turned to Roy Moore to be the special agent in charge of a new field office in the state capital, Jackson. Moore transferred from Birmingham, Alabama, where he had investigated the bombing of a black church in which four young black girls had died. He took more than 150 agents with him to Mississippi.

Nothing in Moore’s career could have prepared him for the challenge of protecting civil rights workers in the South. Born in Oregon in 1914, his early life was spent about as far from the Deep South as was possible for an American child. As a young man he served in the Marine Corps, before joining the FBI in 1938 as a clerk. In 1940 he became an agent, progressing quickly through the ranks. He served as assistant special agent in charge of field offices in Milwaukee, Chicago and Denver. In Denver he was involved in an investigation into the first aircraft bombing in the US, leading to the execution of the bomber Jack Gilbert Graham. By 1960 Moore had been promoted to the “number one man” in charge of training and inspection at FBI headquarters. From there he was dispatched to the hottest spots in the Southern civil rights movement, ending up in Birmingham, and then Mississippi.

In Mississippi Moore determined to break the Ku Klux Klan. He offered one informant $25,000, which led to the discovery of the corpses. His team found that 25 people had been involved in the plot, including two Neshoba County officers. But local law enforcement agencies refused to co-operate. In 1966 Martin Luther King spoke at a rally in Neshoba County, where he complained that “the murderers of Goodman, Chaney and Schwerner are no doubt within the range of my voice”. A voice from a group of white men replied: “Ya damn right. We’re right here behind you.”

In 1967 Governor Paul Johnson Jr — who opposed the Klan — agreed with Moore that the FBI agents should give their evidence to the federal government rather than to the Neshoba County authorities. The federal government tried 19 men for violation of civil rights. An all white-jury found seven men guilty. The suspected mastermind, Edgar Killen, was found not guilty.

Moore was moved to take charge of field offices in Philadelphia, Chicago and the Virgin Islands. At his request, he returned to Mississippi in 1973, retiring the following year. He served as director of security for the Guaranty National Bank until 1982. In 1980 the Mississippi murders returned to prominence during Ronald Reagan’s presidential campaign. Reagan gave his first speech as a candidate at the Neshoba County Fair. He didn’t mention the murders. Instead, he dusted off old segregationist rhetoric and declared: “I believe in states’ rights.” Andrew Young, a former colleague of Martin Luther King, wrote, “One must ask: Is Reagan saying that he intends to do everything he can to turn the clock back to the Mississippi justice of 1964?” The story was the subject of a film, Mississippi Burning (1988). Mississippi prosecutors finally brought Killen to trial in 2005. Aged 82, he was given a 60-year sentence.

Moore’s wife died in 1994. He is survived by two daughters and a son.

Roy Moore, FBI agent, was born on June 11, 1914. He died on October 12, 2008, aged 94