The FBI kept tabs on the late boxer Muhammad Ali in 1966, including his divorce and his speech at a Miami mosque, in its investigation of the religious group Nation of Islam, according to documents released by the agency.

The release of the documents, recently posted on the website of the Federal Bureau of Investigation, was first reported by the New York Times online on Thursday.

The former heavyweight boxing champion died in June at age 74, after a life in the ring and in activism that made him one of the world's most famous celebrities. Former President Bill Clinton was among the dignitaries at his funeral.

The FBI has been criticized for monitoring public figures, including civil rights leader Martin Luther King Jr and rock singer John Lennon, in the turbulent 1960s and 1970s.

The latest batch of about 140 pages of FBI documents from 1966 on Ali, which includes previously classified material, were released following a lawsuit to obtain the papers brought in August by conservative group Judicial Watch.

The papers, which used Ali's birth name Cassius Clay, includes a request for agents to monitor his divorce that year from his first wife as a "lead."

"The Miami (FBI) office is requested to follow the divorce action between Cassius and Sonja Clay with particular emphasis being placed on any NOI (Nation of Islam) implication being brought into this matter," one memo stated.

A separate FBI memo on a speech Ali gave in 1966 at a mosque said he discussed efforts to strip him of his heavyweight title and blamed the "white man."

The controversy centred on Ali's refusal to be drafted into the U.S. military during the Vietnam War and his claim of conscientious objector status, which would lead to his being stripped of the boxing title in 1967. After a successful legal battle, Ali regained the title in a 1974 bout.

In the documents, FBI officials stressed Ali was not personally under investigation.

One memo said Ali's connection to the African-American religious group the Nation of Islam, which was under FBI investigation at the time, made the bureau interested in his activities "from an intelligence standpoint".

In a status report filed in federal court on Dec. 2, Judicial Watch and the FBI said the bureau provided documents on Ali to Judicial Watch in November and anticipated releasing more papers around Dec. 21 and Jan. 21.