Chicago Police issued an alert about a potent drug that is peddled on the streets as heroin, but actually is hundreds of times stronger and may have caused up to a dozen fatal overdoses in recent weeks on the South Side.

The Sun-Times reported Sunday that investigators became concerned about a pattern of fatal overdoses where the victims were either found on a stretch between 27th and 30th, on State and Dearborn, or had bought drugs there. On Monday, Chicago Police officials alerted the public, saying they believe a synthetic drug called fentanyl has been sold as heroin.

Fentanyl, a prescription pain-killer that is 80 times more powerful than morphine and is used by cancer patients, has surfaced before as a substitute for heroin and has proven lethal here and elsewhere the country.

Now, Chicago investigators are working to understand how the drug is hitting the streets.

One suspicion is that dealers are stealing the drug, said Assistant Deputy Supt. Charles Williams, who added that investigators are trying to get more information.

"I know that [the] Narcotics [Section] has a number of people assigned to it," he said, adding fliers were being printed to alert people about the danger of the potent drug.

'Undertaker' and 'Lights Out'

Chicago Police officials did not confirm exact numbers of victims whose deaths were under investigation Monday, but sources have said there are as many as 12 people who fit the pattern. Further toxicological tests must be done to determine whether the victims had ingested fentanyl before overdosing.

Standard toxicological screenings do not detect fentanyl, so special tests must be done.

The drug kills quickly, another reason police noticed a pattern. When just a small amount of the cancer drug is cut into heroin, it can become lethal, according to the U.S. Drug Enforcement Agency. In this case, investigators suspect the victims were ingesting pure fentanyl.

Lawrence Ouellet, a University of Illinois professor and director of the Community Outreach Intervention Projects, said his staff started hearing about overdoses a few weeks ago. They also know of some heroin that has been sold recently under brand names "Undertaker," "Lights Out" and "Overdose." Ouellet said the ghoulish names are not uncommon, and are used to signal how high a person will get from a certain drug.

Illicit use of fentanyl first appeared in the mid-1970s, according to the DEA. Its effects are indistinguishable from heroin, and it can be smoked or snorted.