Ketamine – a powerful anaesthetic – is used as a treatment for depression, anxiety and pain.

People close to Perry, who starred as one of the lead characters on the NBC television show Friends, told a coroner’s investigation after his death that he was undergoing ketamine infusion therapy.

But his last session had taken place more than a week before his death. The medical examiner said the ketamine in Perry’s system could not have been from the infusion therapy because of the drug’s short half-life.

The levels of ketamine in his body were as high as the amount given during general anaesthesia, according to the medical examiner.

An indictment filed in a federal court detailed the elaborate drug purchasing scheme that prosecutors say ultimately led to Perry’s death.

Prosecutors said Perry’s assistant, Kenneth Iwamasa, worked with two doctors to provide the actor with more than $50,000 (£38,000) of ketamine in the weeks before his death.

Officials argued those involved in the scheme tried to profit from Perry’s well-known substance-abuse issues. One of the doctors, Salvador Plasencia, is alleged to have written in a text message: “I wonder how much this moron will pay.”

Dr Plasencia, 42, provided Perry with ketamine “outside the usual course of professional practice and without a legitimate medical purpose”, according to the indictment.

He also allegedly taught Iwamasa how to inject Perry with ketamine without proper safety procedures and surveillance, the indictment said.

In the four days before his death, Iwamasa gave Perry at least 27 shots of ketamine, prosecutors alleged.

He did so even after a large dose of ketamine earlier that month caused Perry to “freeze up”, leading Mr Plasencia to advise against a similar-sized dose in the future, prosecutors said. The doctor still left several vials of the drug with the actor and his assistant after the incident, according to the indictment.

Others charged in the case include Jasveen Sangha, the so-called “Ketamine Queen” who prosecutors allege supplied the drug to Plasencia through the help of two other co-defendants, Erik Fleming and Dr Chavez.