As state police took over the investigation, an attorney for inmate Ronnie L. White's family said the young man's attackers "took it upon themselves to be both the judge, the jury and the executioner."
White's death shocked and angered officials in Prince George's County, including County Executive Jack Johnson, who likened it to "vigilante justice."
Investigators from the FBI and state police joined the case Monday after the state medical examiner concluded that White had been strangled Sunday morning.
White, 19, was arrested early Saturday on charges of ramming a stolen pickup truck into police Cpl. Richard Findley. Findley had gotten out of his police cruiser Friday while trying to conduct a traffic stop on the truck. White allegedly struck him and dragged him for a short distance in the community of Laurel.
Johnson immediately moved to curb speculation that White's death was the result of police officers seeking revenge, saying the slaying was "unrelated to any act" by police.
Attorney Bobby G. Henry Jr., who represented White's family, urged the U.S. Department of Justice to join the investigation.
"This did not happen on some dark, abandoned, lonely road. This happened in broad daylight in the custody of county officials, and something is wrong," said Henry, Jr.
In a statment, the U.S. Attorney's office said it "will provide guidance and legal advice to the officials looking into the death and seeking to develop evidence of criminal violations."
County State's Attorney Glenn Ivey said his office would convene a grand jury to review evidence in the case. He said investigators were scrutinizing seven correctional officers who had access to White, who was being held separately from other inmates. Ivey also said other workers at the county jail and inmates would be questioned.
"We'll follow the evidence wherever it leads," he said.
Rich Wolf, an FBI spokesman in Baltimore, confirmed that the agency had opened a civil rights case, but he would not comment on the investigation. Calls to state police were not immediately returned.
White's death is the latest in a series of security lapses at the jail, which is 20 miles outside Washington in one of Maryland's largest counties. Last month, the county's corrections director was fired after guns vanished from the armory. In February, a former police official convicted of second-degree murder was found with a handcuffs key.
A corrections officer suspected of being a gang member was charged in March with bringing cell phones to inmates who were members of the Bloods street gang. And earlier this year, two female officers were suspended for allegedly having sexual contact with inmates.
More than 450 guards work at the 20-year-old facility, which had an average daily population of 1,489 prisoners during the 2007 fiscal year. White was being held in a cell by himself, to protect him from other prisoners, because of the high profile of his alleged crime.
"This shouldn't happen," Johnson said Monday night. "Every citizen we bring into our custody should be protected."
Curtis Knowles, head of the county's correctional officers union, said Tuesday that union lawyers advised him not to comment on the case because of the investigation. On Monday, he urged people not to reach any conclusions before the investigation had run its course.
Corrections officials have said White had no visible signs of trauma on his body when guards bringing him a meal found him slumped on the floor next to his bed about 10:30 a.m. Sunday. Guards say they say White alive 20 minutes before finding his body and no one reported hearing anything unusual. With no surveillance cameras watching White's cell block, there's no image of the killer entering or leaving White's lockup, reports CBS News correspondent Bob Orr.
A preliminary autopsy by the state medical examiner in Baltimore ruled the death a homicide and found two broken bones in his neck.
None of the seven guards or an unspecified number of supervisors who could have had contact with White have been suspended or placed on leave during the investigation, according to Vicki Duncan, spokeswoman for the corrections division.
White had a prior criminal record that included serving time for drug charges.