The Syrian government killed thousands of people during years of secret mass hangings at a single prison outside the capital of Damascus, an Amnesty International report published on Tuesday alleges.

The report details how authorities at Sednaya prison executed between 5,000 and 13,000 people over a period spanning four years from 2011 to 2015. Interviews with former detainees, prison guards and doctors, as well as other staff, chronicle what Amnesty describes as a “human slaughterhouse.”

The accounts of killings and abuses committed in the prison, which include rape and torture, amount to crimes against humanity, according to Amnesty. The rights group is calling for a United Nations investigation into the allegations and for the body to make sure there is clear evidence the Syrian government has stopped such practices at Sednaya.

Amnesty’s report also brings renewed attention to Syrian President Bashar Assad, at a time when President Donald Trump may shift U.S. priorities in Syria to increasingly focus on fighting militants of the so-called Islamic State and move away from pressuring Assad’s regime. On Tuesday, Assad praised Trump’s rhetoric on terrorism as “promising,” and in November said the new U.S. president could be a “natural ally” against terrorists.

Assad has previously denied the disappearances of prisoners and said that the government doesn’t detain political prisoners, despite overwhelming evidence to the contrary that shows the systematic murder and abuse of Syrian citizens. Amnesty’s new report adds to the long list of human rights accusations against the Syrian government, and also provides a further in-depth look at the already infamous Sednaya facility.

Prisoners were taken in the night from their cells in groups of 20 to 50, and ferried to the basement of one of the jail’s two main buildings. The detainees, mostly political prisoners, were then hanged en masse. The bodies of the victims were brought to a hospital for registration, before being dumped into mass graves on military land. The killings often took place twice a week, usually on Mondays and Wednesdays, according to the report.

Interviewees described to Amnesty that prisoners at Sednaya were subject to phony trials, lasting just one to three minutes, before being executed. The blindfolded prisoners only find out about their death sentences minutes before being killed, having initially been told they were being transferred to a civilian prison. The executions are kept secret from most in the prison and only guards directly involved in the killings, as well as high-up Syrian officials, know about the practice.

In addition to the mass hangings, the report includes horrific accounts of torture and abuse suffered in the prison. It details detainees being deprived of food and water, as well as being subjected to routine beatings and humiliation. Multiple interviewees also told Amnesty that prison officials sexually abused detainees, and forced them to rape each other.

“The guard would ask everyone to take off all their clothes and go to the bathroom one by one. As we walked to the bathroom, they would select one of the boys, someone petite or young or fair. They would ask him to stand with his face to the door and close his eyes. They would then ask a bigger prisoner to rape him,” a former detainee named Omar said in the report. Failure to comply resulted in beatings and torture.

Although the report documents a four-year period up until December 2015, Amnesty says that there is no reason to believe the extrajudicial killings and other abuses have stopped. The group says it’s likely thousands more detainees have been killed in the last year.