Overdose deaths have surged as drugs such as heroin, cocaine and counterfeit prescription pills are commonly laced with fentanyl to increase potency, though drug investigators said it is increasingly sold by itself, too.

The street version of fentanyl blamed in the deaths of thousands of Americans also is threatening police officers, forcing changes in long-standing basics of drug investigations, from confiscations to testing and undercover operations, law enforcement officials say.

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