Local and federal authorities Thursday were investigating the death of a black man in southwest Mississippi who was found hanging from a tree.

Claiborne County Sheriff Marvin Lucas told FoxNews.com that the man, who had been missing since March 2, was found Thursday morning in Port Gibson, near the Louisiana border. But it was not immediately clear whether he committed suicide or was murdered, Lucas said.

Lucas would not confirm the victim's identity. But in a statement, the Claiborne County branch of the NAACP identified him as Otis Byrd, and called for the Justice Department to join the investigation into what the organization called a "suspicious hanging death." MSNewsNow.com, citing the county coroner, also identified the victim as Byrd.

Byrd was convicted in 1980 for the murder of a woman, and served about 30 years in prison before being released about two years ago, Lucas said Thursday night. His body was decomposing when found, and he was hanging from the tree with a sheet for about two weeks, Lucas said.

Local law enforcement found the man about half a mile from his home.

The investigation involves the FBI, the Justice Department's Civil Rights Division and the United States Attorney's office. The Mississippi Bureau of Investigation is also participating.

FBI Supervisory Special Agent Jason Pack said in a statement that the cause and manner of death has not been determined, adding that investigators were processing evidence Thursday afternoon.

Autopsy results were expected to be released early next week.

FoxNews.com's Karl de Vries and The Associated Press contributed to this report.