KANSAS CITY, Mo. (AP) — A former employee of two northwest Missouri ambulance services has admitted that he replaced pain-killing drugs in ambulances with sterile water.

Federal prosecutors say 31-year-old Joseph Comstock pleaded guilty Tuesday to tampering with a consumer product.

The crimes occurred with while he worked for the NTA Ambulance District in Bethany and the Community Ambulance District of Daviess County in Gallatin.

Prosecutors say Comstock emptied vials of morphine and fentanyl for his personal use and replaced them with sterile water.

He told prosecutors he started tampering with drugs in 2014. He also admitted that on at least two occasions, he treated two trauma patients with hip fractures with watered-down fentanyl.