(CNN) -- Dr. Karl Williams says he normally sees three or four deaths from drug overdoses in a typical week as the chief medical examiner in Pennsylvania's Allegheny County, which includes Pittsburgh.
In the past week, he saw 15 -- men and women, of various ethnicities, ranging in age from 22 to 53. All of them appear to have been heroin users who instead received a mix of heroin and fentanyl, a powerful narcotic used to treat cancer patients' pain, Williams told CNN.
"This is pretty clearly somebody manufacturing fentanyl and selling it as heroin," Williams said.
The deaths Williams has recorded are among nearly two dozen in western Pennsylvania linked to a heroin-fentanyl mix, state Attorney General Kathleen Kane said Monday. The "extremely dangerous and potentially lethal" combination has killed 22 people in six counties, Kane said in a written statement.
"We are working with the Allegheny County Police Department, the Pittsburgh Police, and their counterparts in the region to get this deadly mix of heroin off the streets of Western Pennsylvania, and to arrest and prosecute anyone caught selling, distributing, and producing these drugs," her statement said.
Williams said fentanyl can be 10-100 times more potent than morphine, the base molecule in heroin, and it's only distributed as a powder when someone is manufacturing it illegally. The drug is being distributed in bags marked "Income Tax," "Bud Ice" and "Theraflu, Williams said.
"This is not accidental. Somebody is deliberately trying to make a big batch of fentanyl," he said. "It is not an extraordinarily complex molecule to synthesize, and you can find instructions on the Internet. It does not take a sophisticated chemist to do this."
It's not the first time that fentanyl has turned up on the streets of Pittsburgh. In 1988, Williams said, 17 people died when a chemist distributed fentanyl as heroin.
And in 2006, in Philadelphia, 269 people were killed by fentanyl overdoses, the state Department of Drug and Alcohol Programs said. The agency raised new alarms about fentanyl in July, blaming it for 50 deaths in Pennsylvania by mid-2013.
CNN's Matt Smith contributed to this report.