FBI spokesman Scott Wilson told the Associated Press that agents would meet with Levert's family.
The 39-year-old singer died shortly before midnight Sunday, after his breathing became shallow while seated in a restraining chair at the Cuyahoga County Jail's psychiatric ward.
"Our medical people were called and got him to the hospital," Warden Kevin McDonough said.
Family members question the warden's report of Levert's bizarre behavior, the Cleveland Plain Dealer reports.
According to the warden, Levert was moved to the psychiatric ward on the fifth day of his jail stay after he began mumbling incoherently. Once in the ward he became violent and was banging against his cell door, so he was placed in the restraint chair. Authorities say he went in the chair voluntarily, Cleveland's WOIO-TV reports.
Michael Gibson, a first cousin of Levert's, speculates that the singer may have been undergoing withdrawal from anxiety medication.
No immediate cause of death was found in the autopsy conducted Monday, but Cuyahoga County Coroner's Office spokesman Powell Caesar said there was no evidence of foul play or trauma.
Caesar said Levert suffered from high blood pressure and had been hallucinating in jail. Toxicology reports could take four to six weeks, he added.
Levert was serving a 22-month sentence for neglecting to pay nearly $90,000 in child support payments for his three children by two women. He has two additional children with his current wife of 13 years.
"Sean paid child support, just not through the proper channels," Gibson said. "He was a good father."