A former federal agent in the northeastern United States who received such tips from SOD described the process. "You'd be told only, ‘Be at a certain truck stop at a certain time and look for a certain vehicle.' And so we'd alert the state police to find an excuse to stop that vehicle, and then have a drug dog search it," the agent said.

After an arrest was made, agents then pretended that their investigation began with the traffic stop, not with the SOD tip, the former agent said. The training document reviewed by Reuters refers to this process as "parallel construction."

The two senior DEA officials, who spoke on behalf of the agency but only on condition of anonymity, said the process is kept secret to protect sources and investigative methods. "Parallel construction is a law enforcement technique we use every day," one official said. "It's decades old, a bedrock concept."

A dozen current or former federal agents interviewed by Reuters confirmed they had used parallel construction during their careers. Most defended the practice; some said they understood why those outside law enforcement might be concerned.

"It's just like laundering money - you work it backwards to make it clean," said Finn Selander, a DEA agent from 1991 to 2008 and now a member of a group called Law Enforcement Against Prohibition, which advocates legalizing and regulating narcotics.