Zhi Alan Cheng, a former doctor who pleaded guilty to raping and videotaping his patients and co-workers, has been sentenced to 24 years in prison.

Details of his modus operandi were revealed during court proceedings. Alan, who practiced medicine in Queens, New York, would drug his victims, some as young as 19 years old, before committing his crimes.

In one case involving a 19-year-old victim, Alan performed an unnecessary rectal exam before injecting her with an “unknown substance” that rendered her unconscious. He then sexually assaulted her while recording the incident.

During a police raid of his home, authorities seized drugs including ketamine, fentanyl, and other powerful anesthetics classified as controlled substances.

It was also revealed that the hospital had terminated his employment and revoked his medical license. His crimes came to light when a female friend of one of the victims discovered videos showing her and other women being sexually assaulted while unconscious.

In addition to the drugs, police found dozens of videos depicting Alan sexually assaulting sedated women, including patients he treated while on duty and individuals he met through dating apps.

In June this year, Alan pleaded guilty to four counts of first-degree sexual assault and three counts of first-degree sexual abuse, according to the New York Post.

Alan is currently serving his 24-year prison sentence. Upon release, he will be supervised for 10 years and will be registered as a sex offender for life.