New Haven officials warned strongly to the public Friday after almost 20 people overdosed on tainted heroin (a batch of heroin mixed with fentanyl.) at least three were killed in the city.

Most of the cases happened in the city's Newhallville section, police said.

Fentanyl is a synthetic opioid 50 times more stronger than heroin.

Many victims told detectives that they thought they were purchasing cocaine, said Officer David Hartman, police spokesman.

"Any such drug should be considered deadly,” David said. “There is also no reason to suspect there are any boundaries to how far the tainted drugs have been sold or distributed. Neighboring communities are warned as well.”

Deputy Director of Emergency Management Rick Fontana told the New Haven Register the city has a serious shortage of the overdose reversal medication naloxone.

Naloxone blocks or reverses the effects of opioid medication, including extreme drowsiness, slowed breathing, or loss of consciousness. An opioid is sometimes called a narcotic.

Gov. Dannel P. Malloy called this case extremely dangerous.

"Everyone must recognize that no region of the country, state, city or town is immune - this affects all of us and so many families across our state and nation," the Democratic governor said in a statement. "That's why we have been doing everything in our power to stop this epidemic and prevent tragedy." 

"I want cocaine users to be forewarned," U.S. Attorney Deirdre Daly said. "When they think they are buying just cocaine, they are not in these cases. The cocaine that they thought they were buying was, in fact, a lethal opioid."

Police and the federal Drug Enforcement Administration were working to arrange fast testing of seized drugs.