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 several 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 FBI director J Edgar Hoover to get a grip on the state, and 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. 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. Here, Moore became determined to break the Ku Klux Klan. He offered one informant 25000, 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 and 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 jnr - 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.
Moore's story was the subject of a film, Mississippi Burning (1988). Mississippi prosecutors finally brought Killen to trial in 2005. Aged 82, the killer was given a 60-year sentence.
Moore's wife died in 1994. He is survived by two daughters and a son. Born on June 11 1914, he died on October 12 2008, aged 94.