U.S. anti-drug czar John Walters said Monday that federal agents, working in cooperation with the Mexican government, have closed down a lab in Mexico that might be the main source of the powerful painkiller fentanyl, which has killed heroin users in eight states.

Walters, the director of the White House Office of National Drug Control Policy, said it's still not clear whether the fentanyl was mixed with heroin at the lab in Mexico or after it entered the United States. Fentanyl-laced cocaine also has turned up in some cities, he said.

He warned drug users that millions of deadly doses of fentanyl-laced heroin might still be on the streets. The mixture has caused at least 100 confirmed deaths from Philadelphia to Chicago in recent months. Fentanyl might also be coming from other sources, he said.

"There may be more than one source," Walters said. "We think this is the principal source."

He referred specific questions to the U.S. Drug Enforcement Agency, which declined to provide details immediately.

Walters said the bust occurred in May.

Deaths from fentanyl-laced drugs have occurred in Illinois, Michigan, Missouri, Ohio, Pennsylvania, New Jersey, Delaware and Maryland, Walters said.

The bust led to the arrest of five individuals, including a person Walters described as "the chemist." He said the size of the fentanyl operation made the bust extremely significant.

He said the fentanyl-laced heroin might have been used by dealers looking for a competitive advantage on the street, but inept mixing — or cutting — of the drug into heroin made it deadly.

Walters was in Chicago to release a new series of anti-methamphetamine advertisements in Spanish and English.