Assistant U.S. Attorney John D.R. Atchison was found unresponsive, taken to a local hospital and pronounced dead at 10:17 a.m., Federal Bureau of Prisons spokeswoman Felicia Ponce said.
Atchison, of Gulf Breeze, Fla., was being held in a special housing unit in the prison, about 36 miles southwest of Detroit.
The administrative detention area houses all levels of prisoners, and Atchison had a cell to himself, Ponce said.
An investigation into his death is under way, she said.
He was charged with three felonies, the most serious of which was crossing state lines with intent to have sex with someone under 12. A conviction had carried a minimum of 30 years in prison.
A statement released Friday by his lawyer, James Thomas, said Atchison had "done a lot of good in his life."
"Unfortunately, he is going to be judged by his most recent charges and what we have read in the media, and not by the goodness, hard work or by the love of his family."
The statement also said Thomas would file a request to have the case dismissed.
Atchison had attempted to hang himself with a bedsheet last month in a Sanilac county jail, but another inmate alerted guards who stopped the suicide attempt. He later was taken by U.S. Marshals to the Milan prison.
Three days before that suicide attempt, Atchison had assured a judge he would not harm himself.
The married father of three was a respected figure who coached girls' softball and basketball in a park a few blocks from his Gulf Breeze home. His office was in Pensacola.
But authorities say he had been communicating with an undercover sheriff's detective from Macomb County, Mich., who was posing online as the fictitious girl's mother and arranged for him to have sex with the child.
He was arrested Sept. 16 at Detroit Metro Airport carrying presents, including a doll and earrings, and sexual materials, officials said.
As a federal prosecutor, Atchison mostly worked on tax and financial crime cases. According to court records, it appears he never prosecuted child sex abuse cases.