The federal Drug Enforcement Agency has sent a letter to law enforcement officials across the country warning of a nationwide spike in fentanyl-related mass-overdose events. The DEA raised the alarm following a surge of nearly 30 fatalities in at least seven cities in recent months, as drug traffickers continue to push fentanyl and mix the powerful synthetic opioid with other illicit drugs. Mass-overdose events—defined as three or more overdoses occurring close in time and at the same location—have taken place in cities including Austin, Texas, Omaha, and Washington, D.C. The usual scenario, the agency says, is when drug dealers sell their product as “cocaine,” when it contains fentanyl, or when they sell pills designed to appear nearly identical to legitimate prescriptions but are actually fake pills containing pills—a frightening trend where many victims are dying after unknowingly ingesting fentanyl. The warning comes as fentanyl poisoning fuels the nationwide opioid epidemic that last year killed more than 100,000 Americans. In response, the DEA says it is working to trace mass-overdose events back to the local drug trafficking organization and international cartels responsible for the surging domestic supply of the drug.