Los Angeles jail guards have frequently punched incarcerated people in the head and subjected them to a “humiliating” group strip-search where they were forced to wait undressed for hours, according to a new report from court-appointed monitors documenting a range of abuses.

The Los Angeles sheriff’s department (LASD), which oversees the largest local jail system in the country, appears to be routinely violating use-of-force policies, with supervisors failing to hold guards accountable and declining to provide information to the monitors tasked with reviewing the treatment of incarcerated people.

The report, filed in federal court on Thursday, adds to a long string of scandals for the department. The monitors – first put in place in 2014 to settle a case involving beatings – suggested that some problems in the jails appeared to be getting worse after they visited the facilities in December 2021.

The monitors, Robert Houston, a former corrections official, and Jeffrey Schwartz, a consultant, alleged that the use of “head shots”, meaning punches to the head, had been “relatively unchanged in the last two years or more, and may be increasing”. They also wrote that deputies who used force in violation of policy were at times sent to “remedial training” but that “actual discipline is seldom imposed”. And supervisors who failed to document violations were also “not held accountable” .

The authors cited one incident in which a deputy approached a resident who had “walked away from him” while he was being escorted. “With no hesitation, Deputy Y grabbed [his] chest and slammed him into the wall. Deputy Y punched [him] 5‐9 times in the head, and Deputy Z punched [him] 6‐8 times in the head as they took [him] to the floor because they ‘feared’ that the Inmate might become assaultive”.

The report also documented an incident on 7 September 2021, when there were reports that a firearm “might have been smuggled” into Men’s Central jail. Guards responded by instituting a “shakedown” and strip-search of residents.

“They said they were taken out of their cells in the morning, given no explanation (except for one inmate who said he was told the reason for the search by a deputy), strip-searched, then walked naked en masse through the jail and down to the room with the X‐ray machine,” the report said, citing complaints from jail residents. “Passing large numbers of male and female staff members, some of whom … mocked them or made other humiliating comments”.

Those interviewed said they eventually got underwear, but still no shoes, and were taken to a yard where they were forced to wait for hours until they returned to their cells later that night.

LASD did not immediately respond to a request for comment about the report on Friday.

The monitors said they had written officials in January to ask if this was standard procedure, and whether the residents were given food, water and access to bathrooms while waiting. According to the monitors, the department responded that it had completed a “report” about the incident with “corrective action plans”, but in the three months since, it had not sent documents or further information.

The report raised further concerns about the department’s use of the “Wrap” device, which functions like a full-body restraining jacket and is used to “immobilize” people. The Wrap procedures pose a serious risk of asphyxiation, and “the continuing practice … cannot be justified”, the monitors said.

The department had failed to fulfill its requirement to write a Wrap policy that the monitors had approved, and it had further misled the monitors about how the jail was using the device, the report alleged: “The practices used with Wrap appear to be almost diametrically opposed to the way in which the Department explained that Wrap was being used.”

In 2018, a man in jail in northern California died of asphyxiation after being subjected to the Wrap device, sparking widespread scrutiny of the practice.

The LA jails have for years been plagued by corruption and obstruction of justice scandals, with the former sheriff Lee Baca and his second in command both convicted in cases stemming from misconduct investigations. Guards in the Men’s Central jail have also long been accused of being part of a “deputy gang”, known for allegedly using excessive force. The department has also faced mounting questions this year about the death of a 27-year-old in solitary confinement.

“These are not one-time incidents – this is the culture and history of the department,” said Mark-Anthony Clayton-Johnson, executive director of Dignity and Power Now, a group that has long been fighting to shut down the Men’s Central jail. He said the report reminded him of the misconduct allegations and obfuscation from department leaders in a 2012 case. “After 10 years of exposure, 10 years of scandal, 10 years of reform, this department has had a lot of opportunities to get this right … but has continued to revert back to some of the most vicious attacks on Black and brown people.

“It is clear our loved ones are not safe in the custody of the sheriff’s department,” he added.

Peter Eliasberg, chief counsel at the ACLU Foundation of Southern California, said it was especially disturbing that the problems seemed to be escalating under sheriff Alex Villanueva, who was elected in 2018: “They are treating incarcerated people in the jails in a sub-human manner … There’s just an utter lack of accountability, which ultimately goes to the top.”

Helen Jones, an organizer whose 22-year-old son died in LA sheriff’s custody in 2009, said she wasn’t surprised by the report: “It’s been this way for so long, it’s just the norm. It’s out of control, and there are no consequences.”