Tuggle, the DEA leader, said authorities in other states have arrested some distributors. That could help advance the Maryland probe, he said.

Tuggle said his agents will study each of the reported overdose cases for possible patterns.

Rosenstein said authorities are poised to start digging into any new cases, which will be easier than working older deaths. And agents could also seek to interview users who have overdosed and survived.

Authorities also plan to examine recovered heroin for impurities that could reveal whether the fentanyl is being stolen from medical facilities, or whether it is being manufactured in illicit labs.

Authorities have had success tracking the labs during previous outbreaks, and those cases suggest that investigators might have to look far afield.

In 2006, 36 people in Maryland died using the combination, according to the Department of Justice. The Drug Enforcement Administration described that wave as a "health crisis for both users and law enforcement."

After Mexican authorities arrested five people at a lab the DEA believed was producing much of the fentanyl then in circulation, the deaths tapered off.

In the early 1990s, a strain known as "China White" was tied to around 30 deaths in Maryland. Federal authorities were able to track down members of a crew, including Gioia's client, that had been distributing it and secure life sentences for seven defendants.

That outbreak was halted after agents in Wichita, Kan., interviewed an overdose survivor, leading to the arrest of a 47-year-old chemist who was synthesizing fentanyl and marketing it with a partner.

The current fentanyl-releated deaths are part of an overall uptick in fatal drug overdoses: There were 378 in 2012, up from 245 the year before.

Tuggle blamed the increase in part on growing numbers of drug users switching from prescription opiates such as oxycontin to heroin.

Tuggle worries that this new wave of users, unlike the savvy customers that Robarge described, might not be as aware of the risks street heroin poses.

"This problem could very easily exacerbate itself," he said.