The Justice Department said on Thursday that it was investigating the conditions at a jail in Fulton County, Ga., citing reports of violence, deteriorating surroundings and the death last year of an inmate who was covered in lice and feces.

The civil investigation, part of a broader effort by the department to scrutinize conditions at jails and prisons across the country, will also examine whether officers used excessive force, the availability of medical care and the treatment of mentally ill prisoners.

“Detention or incarceration in jail should not include exposure to unconstitutional living conditions that place lives in jeopardy or risk of serious harm from assaults,” Kristen Clarke, an assistant attorney general who leads the agency’s civil rights division, said in announcing the investigation. “Jail facilities must provide constitutional and humane conditions, in which all people can live safely while they go through the criminal process.”

The Fulton County government and sheriff’s office said in a joint statement that they planned to cooperate fully.

Conditions at Fulton County Jail have been the subject of criticism for years. The jail was under federal supervision from 2006 to 2015 after a court found that the detention complex was overcrowded, understaffed and dangerous. Exceedingly cramped conditions in 2020 led an expert on infectious diseases to warn of a mass outbreak of the coronavirus at the jail unless it drastically reduced its population size.

In her announcement, Ms. Clarke said there was “significant justification” to open the inquiry, including acknowledgment from local law enforcement of the jail’s dilapidated state and a “deeply concerning” level of violence where the jail averaged more than one stabbing per day at one point last year. Inmates at the jail are often people of color, she added, saying that 87 percent were Black.

She also pointed to the death of an inmate, LaShawn Thompson, 35, in September. Mr. Thompson, who had been arrested on a battery charge, was kept in a filthy cell in the jail’s psychiatric ward and died after weeks of severe neglect, a private autopsy conducted on behalf of his family said.

The medical examiner found that Mr. Thompson was malnourished and dehydrated, had lost 32 pounds in less than 90 days, and had matted hair, dirty nails and “innumerable” insects all over his body. The examiner also wrote that Mr. Thompson, who had schizophrenia, had not received medication for his condition for over a month.

Advocacy groups welcomed news of the Justice Department’s investigation.

Fallon McClure, the deputy director of policy and advocacy at the American Civil Liberties Union of Georgia, said in an interview that the Georgia branch had recommended that the jail reduce its detainee population in a September report.

Ms. McClure pointed to steps taken at a jail in neighboring Cobb County, where a smaller population allowed for cell blocks to be renovated. “It’s a lot easier to make the conditions better if you’re not overcrowded,” she said.

The Southern Center for Human Rights, a nonprofit public interest legal firm that wrote to the Justice Department in April about Mr. Thompson’s case, described the investigation as a step forward.

Its executive director, Terrica Ganzy, called the inquiry “a significant step toward a reckoning for the lives tragically and senselessly lost, and for the many people who continue to suffer rampant indignity and abuse in Fulton jails.”

At least four officers at the jail have been arrested or fired for misconduct this year. One case involved a detention officer who now faces felony charges of aggravated assault and cruelty to inmates.

The jail also opened its doors to the local news media this spring to highlight the poor conditions, including sections that were unusable, bedding shortages, water leaks and plumbing issues, rusting cell doors, large holes in the walls and staffing shortages.

In May, an inmate at the jail tunneled through a wall to an adjacent cell block to stab another prisoner, jail officers said, prompting a search of both cells and the seizures of weapons, including “shanks made from parts of the dilapidated building infrastructure.”