The department said in a letter to county officials it is "increasingly concerned" about reports that deputies and jail staff were using excessive force on inmates, and would also look into the lack of housing and care for mentally ill prisoners.
"A growing number of prisoners with serious mental illness continue to be housed in obsolete and dilapidated conditions at Men's Central Jail," the letter stated.
The sheriff's department is already under FBI scrutiny for similar allegations. The Justice Department probe is separate from the bureau's investigation, the Los Angeles Times said.
The newspaper said the department last year was criticized by a blue-ribbon county commission which concluded there was a culture of "jailhouse justice" in which overly aggressive deputies were able to use beatings and humiliation of inmates with little fear of being disciplined.
Steve Whitmore, a spokesman for Los Angeles County Sheriff Lee Baca, said the department will cooperate with the federal investigation but he said the department questions the use-of-force allegation and has taken steps to deal with the matter.
"It is being tracked now like it's never been tracked before," he said. "Use of force is down by more than half."