Police in the Atlanta area say a number of deaths in the past 18 months have been linked to unusually potent Mexican heroin.

Mitch Plumb, a detective in Smyrna, Ga., said the heroin appears to come from the Midtown entertainment district in Atlanta.

Two men died in Smyrna in January after buying what they thought was cocaine, which turned out to be heroin, police said. Their girlfriends, who also took the drug, survived but became sick.

"It's been going on for a year and a half now, it's still going on and it's been traced to Midtown," Plumb told The Atlanta Journal-Constitution.

"Heroin in this area is becoming very toxic. What's odd about these cases is they're resulting in double deaths, which is unusual. We're finding out that a lot of these probably shouldn't be overdoses."

Plumb said he discovered other deaths as he continued his investigation. In one case, an overdose victim woke up after two days in a coma to find his two companions dead.