The family of a former N.F.L. player is raising questions about his death this month while in the custody of a sheriff’s office in Alabama, saying the initial findings of an autopsy that they commissioned suggested he had not died from “natural causes.”

The former player, Glenn Foster Jr., 31, who had a history of mental health issues, died on Dec. 6, in the custody of the Pickens County Sheriff’s Office, days after he was arrested while speeding, lawyers for the family said.

“Glenn Foster Jr.’s death, while in the Pickens County Sheriff’s custody and care, was not from natural causes as the independent autopsy suggests there was some evidence of neck compressions and strangulation,” the lawyers said in a statement.

“As we continue to investigate the case, we are learning that Mr. Foster’s death in Pickens County appears to be part of a disturbing trend of Black men dying while in the custody of the Pickens County Sheriff’s Office,” the statement said. It did not cite other cases.

The Pickens County Sheriff’s Office directed questions on Friday to the State Bureau of Investigation. The bureau referred to a statement it released on Dec. 7 confirming that it was investigating Mr. Foster’s death, which had been reported on Dec. 6 at a medical facility in Northport, Ala.

The statement said that no further information was available and that the agency’s findings would be turned over to the Tuscaloosa County District Attorney’s Office.

The Alabama Department of Forensic Sciences did not immediately respond to a request on Friday for the results of Mr. Foster’s official autopsy. The lawyers for Mr. Foster’s family said they had not been provided with the official autopsy.

Mr. Foster played in 17 games in 2013 and 2014 as a defensive end for the New Orleans Saints. He was also a husband, a father of four and a businessman, the lawyers said.

“Our son was a pillar in the community, not only here in the New Orleans area, but also in the Chicagoland area,” his father, Glenn Foster Sr., said at a news conference over the weekend, before the release of the initial findings of the private autopsy. “It’s a tragedy that his life was cut short and under, I would say, suspicious circumstances.”

Mr. Foster had been arrested after the police in Reform, a small town in western Alabama, clocked him on radar driving 92 miles per hour in a 45-m.p.h. zone at about 11:10 p.m. on Dec. 3, the police said.

After the police pursued Mr. Foster in his Jeep and placed spike strips on the road, he was charged with reckless endangerment, attempting to elude and resisting arrest, the police said. He was also cited for speeding, driving while suspended and driving on the wrong side of the road, the police said.

Mr. Foster’s parents believe he may have been experiencing a “manic episode” in the days leading up to, during and after his arrest, the family’s lawyers said. Mr. Foster had “a history of mental health challenges,” the lawyers said in a statement.

While Mr. Foster was being booked into the Pickens County Jail, he was “not cooperative in answering the jailer’s booking questions but there was no physical altercation during the booking process while Reform Police Department was present,” the department said in a statement.

On Dec. 5, Mr. Foster’s family posted his bond, which included a condition, by judge’s order, that the police take him to a hospital in Birmingham for an assessment, the police said.

But when Mr. Foster’s family and the police arrived at the jail to take him to the hospital, the sheriff’s office declined to release him, saying he had been rebooked on another set of charges, the Reform Police Department said.

According to the lawyers for Mr. Foster’s family, they had been told that Mr. Foster had “been in an altercation with another inmate and was now under the jurisdiction of the sheriff’s office, which delayed his treatment.” The family was not allowed to visit Mr. Foster, the lawyers said.

A New Orleans television station, WWL-TV, reported that court records it had obtained indicated that Mr. Foster had been accused of attacking a sleeping inmate in jail while trying to steal his socks.

Mr. Foster died on Dec. 6 “after being found unresponsive in the back of a police cruiser upon arrival at a medical facility,” his family’s lawyers — Diandra Debrosse Zimmermann, Robert F. DiCello and Ben Crump — said.

Mr. Crump has also represented the families of Trayvon Martin, Ahmaud Arbery and George Floyd. The autopsy, the lawyers said, was performed by Dr. Michael Baden, who also participated in a private autopsy of Mr. Floyd.

Dr. Baden has been a consultant on other high-profile cases.

In 2019, after he was hired by the brother of the disgraced financier Jeffrey Epstein, he disputed the official finding that Mr. Epstein died by suicide, saying the evidence “points to homicide.” The New York City medical examiner’s office had concluded that Mr. Epstein had hanged himself in his jail cell while awaiting trial on sex-trafficking charges.