LOWELL (CBS) – Four people died of drug overdoses involving fentanyl during a 12-hour period, the fire chief said Thursday.

“We believe these overdoses were a mix of heroin/fentanyl and cocaine/fentanyl,” Lowell Fire Chief Jeffrey Winward said in a statement on Twitter.

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“There appears to be a lethal batch of opiates in the greater Lowell area,” Winward said. “We strongly urge people not to use opiates, but if you or your loved one is suffering from addiction, please be extra vigilant in the coming days.”

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While visiting Haverhill on Tuesday, Gov. Charlie Baker called for tougher laws on the dangerous drug fentanyl, a problem that is plaguing Massachusetts.