The FBI and Maryland State Police are investigating the strangulation of a 19-year-old man found dead in his cell a day after he was jailed on charges of running over and killing a police officer, authorities said.

Ronnie White's death was a homicide, the Maryland Medical Examiner ruled Monday. He died Sunday in the Prince George's County Correctional Center from asphyxiation and strangulation, the examiner said.

State police are leading the investigation into White's death at the request of county officials. The FBI is focusing on possible civil rights violations.

Officials said seven guards had access to White at the time of his death, as did an unspecified number of supervisors. Authorities are also investigating whether anyone from the outside had access to the inmate.

"If we have vigilante justice, our society will fall apart," Prince George's County Executive Jack Johnson said at a news conference Monday.

Johnson said that he believes the death was "unrelated to any act by the Prince George's County Police Department."

Curtis Knowles, president of the corrections officers' union, said he was working that day but not in the unit. Officers involved in monitoring the inmate told Knowles that they came to feed White, tapped on the window and yelled through a slot in the door, but White didn't reply. They went in and shook White, but he didn't respond, Knowles said.

Knowles also cautioned that the investigation needs to take its course before any conclusions are reached.

"We're not going to speculate. We're going to depend on the FBI and the state police," Knowles said.

June White-Dillard, head of the Prince George's branch of the NAACP, said that she was pleased the FBI was investigating the case but disturbed by the nature of the death of the inmate.

"No one should have had access to him that could have caused him harm," she said.

White was found on the floor of his cell at the Prince George's County jail at 10:30 a.m. Sunday with no pulse, according to officials. Jail medical staff who treated him reported no visible signs of trauma on his body, and he was declared dead an hour later at a hospital.

White had been held by himself in maximum security since he arrived at the jail around 12:30 a.m. Saturday morning.

He was charged with first-degree murder in the death Friday of Prince George's County Cpl. Richard Findley, 39, during a traffic stop in Laurel. Findley, who was part of a team investigating car thefts, was killed after he got out of his cruiser and was dragged by a truck, which had been reported stolen. Authorities said White was driving the truck.

Gov. Martin O'Malley ordered the state flag be flown at half-staff until sunset Thursday in memory of Findley, a 10-year veteran of the county police.

Prince George's County State's Attorney Glenn Ivey said Monday that his office would review the circumstances of White's death.

White was one of four people taken into custody by police after the truck was found at a nearby apartment complex shortly after Findley was killed. The three others were questioned and released, according to Officer Henry Tippett, a county police spokesman.

White received medical and psychological assessments that jail officials said did not uncover any problems. He was placed in a maximum security cell because he was considered a "high profile offender."

Guards checked him every half hour. At 10:15 a.m. he was sitting on the side of his bunk and was alert, according to a timeline provided by jail officials. But fifteen minutes later, he was found unresponsive.

There have been several problems recently at the county jail, including a former police corporal convicted of murdering a man who was found with a handcuffs key. The county's director of corrections was fired in early June when four handguns went missing from the jail armory.

According to court records, White pleaded guilty to illegal possession of a firearm last year, and in another case, to drug possession. In 2006, he was charged with first-degree assault and armed robbery, but the case was dropped, records show. Last November, he was sentenced to more than six months in prison, but it's not clear when he was released.

Findley was also a volunteer firefighter in the county, according to Vince Canales, president of the police union. A viewing was scheduled for Wednesday, followed by a funeral Thursday in Beltsville. Findley is survived by a wife and two young daughters.

"He was a wonderful person," said Al Schwartz, chief of the Beltsville Volunteer Fire Department where Findley served for 20 years. "He could make you smile in a heartbeat."