A group of 18 people allegedly involved in fentanyl and methamphetamine trafficking in Fresno County were arrested after a string of overdoses, including two deaths, federal and local officials announced Friday.

The arrests were made Thursday during a multi-agency operation that included law enforcement from the Fresno Police Department, Fresno County Sheriff’s Office, Fresno County District Attorney’s Office and the Department of Justice, The Fresno Bee reported.

The investigation began after a series of fentanyl-pill overdoses in the Fresno area caused by counterfeit oxycodone M30 tablets containing fentanyl, referred to on the street as M30s, the U.S. Attorney’s Office said.

Similar to authentic oxycodone M30 tablets, they are small, round, and light blue or green in color with “M” stamped on one side and “30” on the other.

The group obtained tens of thousands of counterfeit M30 fentanyl pills and large quantities of fentanyl powder, cocaine and methamphetamine from Mexico and distributed the drugs to dealers in California and other places, authorities said.

According to the most recent Fresno County Coroner statistics, fentanyl was the cause for the second-highest number of drug overdose deaths at 17 deaths in 2020. The number one cause was methamphetamine at 121 deaths, the newspaper reported.