SAN CARLOS — A felon has been arrested after purportedly working as an unlicensed dentist and cooking meth at his “crude” San Carlos office, authorities say.

Joseph Hirsch, 59, is being held in San Mateo County jail on $500,000 bail on suspicion of offenses that include possessing and manufacturing controlled substances, possessing brass knuckles, and possessing ammunition as a felon.

Agents with the San Mateo County Narcotics Task Force arrested Hirsch on Tuesday after a three-month investigation that grew to involve state Department of Dentistry investigators, who work under the state Department of Justice. But the site of the arrest had been under suspicion since January 2016 when someone tipped off authorities.

They served a search warrant at Thermo Dental, in the 600 block of Old County Road, located in an industrial area of San Carlos across from the city’s Caltrain station. The site is flanked by auto-repair shops, textile wholesalers and other warehouse businesses.

“As a result of this search, narcotics and equipment commonly used to manufacture narcotics” were seized, according to a news release from the task force. The news release did not specify the drug involved, but the warrant affidavit mentioned methamphetamine.

Agents also discovered what they described as a “crude dentist office” complete with a dentist’s chair, x-ray machine and dental tools.

“It is suspected that Mr. Hirsch was providing dental treatment to various patients without a license,” the news release said.

What wasn’t clear was whether any patients were injured as a result of their treatment, or how long Hirsch was puportedly treating people. An investigator with the Department of Dentistry did not immediately return a request for comment.

A man who answered the phone at Thermo Dental, which has been operating for about six years according to public records, said he had no comment about Hirsch’s arrest.

Additional details were not immediately available. Anyone with information about Hirsch or for the investigation can contact the county narcotics task force at 650-573-3991.