Happening now:

Law enforcement sources tell us ... the ketamine Perry ingested was not legally prescribed and multiple agencies, including the DEA, U.S. Postal Service and LAPD, have conducted a criminal investigation for months.

We're told at least 1 doctor has been arrested, along with several dealers who helped arrange and deliver ketamine to Perry.

Our sources say the search warrant revealed text messages discussing Perry and the ketamine he wanted and what they were doing to get it to him. The texts also discuss the price the "Friends" actor would pay for the drug.

Perry was found dead last October in the hot tub of his home. He had routinely struggled with addiction.

Ketamine is an anesthetic used during surgical procedures. It is also sometimes prescribed for depression and pain management.

Sources tell TMZ, authorities got a search warrant for Brooke Mueller, Charlie Sheen's ex, who had been in a treatment center with Perry, but as far as we know she is not connected to the arrests. In their investigation, authorities found other celebs who were involved in the Hollywood drug scene.

More from Fox News:

We'll update you with any more info on the case.

P.S. Now check out our latest video 👇