Fentanyl is quickly becoming America’s deadliest drug. But law enforcement couldn’t trace it to its source — until one teenager overdosed in North Dakota. Around 3 a.m. on Saturday, Jan. 3, 2015, Laura and Jason Henke awoke with a start at their home in Minot, N.D. Their dog was barking wildly. At the door, in the early morning shadows, they found a police officer and, behind him, a pastor. The officer asked to see Laura’s ID to confirm that he was at the correct address. Then he told them that their 18-year-old son, Bailey, was dead. The officer didn’t have many details. Bailey Henke was living in Grand Forks, three hours east of his parents’ home in Minot, and the police there were working the case. The officer gave Laura the phone number for a detective in Grand Forks. She called and wrote down what he said: overdose, fentanyl. Laura had never heard of fentanyl; she wasn’t even sure how to spell it. After a few minutes, the officer and the pastor left. A heavy snowstorm had closed the roads, leaving Laura and Jason unable to reach Grand Forks that night. They spent the dark hours sitting on the couch, waiting for the storm to clear, moving in and out of spasms of inconsolable...