Illicitly manufactured fentanyl is a key factor driving opioid overdose deaths, according to a new report by the Centers for Disease Control and Prevention (CDC).

Fentanyl and similar drugs, such as carfentanil, are increasingly contributing to a complex illegal opioid market with significant public health implications, the CDC said.

The CDC analyzed toxicology reports from almost 5,200 fatal opioid overdoses in 10 states between July and December 2016. They found fentanyl and similar drugs were directly responsible for more than half of the opioid overdose deaths, HealthDay reports.

In most cases, fentanyl or similar drugs were mixed into heroin, often without the knowledge of the people who overdosed. In almost half of fatal overdoses involving fentanyl, the drugs were injected. Fatal overdoses also occurred when drugs were swallowed or snorted, the CDC said.