Fentanyl combined with a sedative are believed to have caused last week’s mass overdose in Baltimore’s Penn North neighborhood, federal scientists said.

Two survivors are on the long road to recovery at a West Baltimore behavioral health facility, a week after a mass overdose incident rocked the intersection of Pennsylvania and North Avenues.

To get to the bottom of what exactly was in the drugs, the city sent samples to a lab at the National Institute of Standards and Technology.