A 17-month federal investigation has found numerous abuses at the nation's largest single-site county jail, including a failure to protect prisoners from being harmed by other inmates and staff, officials said Thursday.

Cook County Jail inmates were not only unprotected from excessive force by staff and violence from fellow prisoners but the jail failed to provide adequate medical and mental health care, fire prevention and sanitation, the investigation found.

"The Cook County Jail has an obligation to provide conditions of confinement that do not offend the Constitution and take reasonable measures to protect inmates from harm," U.S. Attorney Patrick J. Fitzgerald said. "The investigation clearly found that the jail failed that test."

Fitzgerald planned a news conference to discuss findings outlined in a 98-page Justice Department letter detailing the alleged abuses. It was given to Cook County Board President Todd Stroger and Sheriff Tom Dart a week ago, federal officials said.

While it found many faults with the jail, the letter commended jail staff for cooperating during the civil investigation.

The Cook County Jail sits on 96 acres on Chicago's West Side and has a daily population of about 9,800 male and female inmates, most awaiting trial in the state's criminal court system, making it the nation's largest single-site county jail, federal officials said.

Federal officials said inmate violence in 2006 resulted in the death of two prisoners. They said that in one week in March 2007, the jail documented 35 inmate fights and the use of force to stop them was required in 27 instances. Guards confiscated 46 weapons from inmates.

The report said inmates are regularly subjected to inappropriate and excessive use of physical force. The investigation found such force has sometimes been used after the inmates already were restrained.