Updated

Three more people were declared dead Sunday from apparent heroin overdoses in Pittsburgh, a day after a county health official reported that 14 people had died from heroin overdoses in the past week.

Allegheny County Medical Examiner Karl Williams told the Pittsburgh Post-Gazette on Sunday that the growing number of apparent heroin overdoses is a "major public health crisis."

Williams said that preliminary testing of bags found at the scene of the overdoses and stamped with the word "Theraflu" appear to show that they contain fentanyl, a synthetic morphine substitute that is roughly 100 times more powerful than morphine.

It resembles heroin, though it is much more potent, and it has been blamed for dozens of deaths around the U.S.

Williams said he does not know whether the substance believed to be involved in the overdoses is prescription fentanyl or "something synthesized by some clever chemists."

Each of the people who died in the rash of overdoses has a known history of drug abuse, particularly heroin, Williams said.

On Saturday, Williams' office said 14 people in Allegheny County had died from heroin overdoses in the past week. Deaths also have been reported in Westmoreland County, Pittsburgh Mayor Bill Peduto's office said.