The Drug Enforcement Administration has joined an investigation into the source of a batch of heroin that killed 22 people in western Pennsylvania, The Wall Street Journal reports. The heroin involved in some of the deaths contained the synthetic opiate fentanyl, often used during surgery.

“We do have a good idea where it’s coming from,” Pennsylvania Attorney General Kathleen Kane said of the drug mix. “We’re trying to find the source and get them off the street before there are any more deaths.”

Dr. Karl Williams, medical examiner for Allegheny County, said 15 overdose deaths in the county appeared to be linked to heroin and fentanyl. In an average week, there are five overdose deaths in the county. More nonfatal overdoses were also reported.

Officials found bags of heroin mixed with fentanyl at the scene of overdose deaths stamped with the names “Theraflu” and “Bud Ice,” the article notes. While most heroin is a tan color, these bags of powder were pure white, Williams said. “Clearly, someone has mixed up a big dose of it,” he said.

Some local law enforcement and health officials are concerned that warning drug users about the dangerous heroin mix will encourage them to seek it out for a more potent high. “A lot will chase it, and demand goes up,” Neil Capretto, Medical Director of Gateway Rehabilitation Center told the newspaper. “They will think those who died were just careless.”