Public health officials in King County have issued a warning about an increasing number of overdoses caused by fentanyl found in black tar heroin.

The King County Medical Examiner’s Office has identified two fatal overdoses since March where either black tar heroin or a fake black tar substance contained fentanyl, KOMO-TV reported.

The mix of fentanyl and heroin has not been seen in King County until recently, but has contributed to overdoses in other parts of the country, county health officials said.

“We’ve been very concerned that if this combination started to show up in the local drug supply, it could lead to an even greater increase in the drug overdose epidemic,” county health strategic adviser Brad Finegood said, adding that the few cases they have seen are concerning for the region.

The combination is concerning because people may not know the drug they are taking is laced with fentanyl, officials said.

Public health officials advise people to immediately call 911 if someone is having an overdose.