A pipe was the only sign of drug use found near Chris Bennett's body in November. But it looked like the 32-year-old Taunton, Mass., native had stopped breathing and died of an opioid overdose. Bennett's mother, Liisa, couldn't understand what happened. Then she saw the toxicology report. "I'm convinced he was smoking cocaine that was laced," she says. "That's what he had in his system, [it] was cocaine and fentanyl." Liisa Bennett was shocked. Chris had developed an addiction to pain pills and then heroin in his late teens but had not used opioids for at least 10 years, as...