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.
(Read more: NSA chief on hecklers: 'This is an emotional issue')
"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.
Some defense lawyers and former prosecutors said that using "parallel construction" may be legal to establish probable cause for an arrest. But they said employing the practice as a means of disguising how an investigation began may violate pretrial discovery rules by burying evidence that could prove useful to criminal defendants.
A question of constitutionality
"That's outrageous," said Tampa attorney James Felman, a vice chairman of the criminal justice section of the American Bar Association. "It strikes me as indefensible."
Lawrence Lustberg, a New Jersey defense lawyer, said any systematic government effort to conceal the circumstances under which cases begin "would not only be alarming but pretty blatantly unconstitutional."
Lustberg and others said the government's use of the SOD program skirts established court procedures by which judges privately examine sensitive information, such as an informant's identity or classified evidence, to determine whether the information is relevant to the defense.
"You can't game the system," said former federal prosecutor Henry E. Hockeimer Jr. "You can't create this subterfuge. These are drug crimes, not national security cases. If you don't draw the line here, where do you draw it?"
Some lawyers say there can be legitimate reasons for not revealing sources. Robert Spelke, a former prosecutor who spent seven years as a senior DEA lawyer, said some sources are classified. But he also said there are few reasons why unclassified evidence should be concealed at trial.
"It's a balancing act, and they've doing it this way for years," Spelke said. "Do I think it's a good way to do it? No, because now that I'm a defense lawyer, I see how difficult it is to challenge."
Concealing a tip
One current federal prosecutor learned how agents were using SOD tips after a drug agent misled him, the prosecutor told Reuters. In a Florida drug case he was handling, the prosecutor said, a DEA agent told him the investigation of a U.S. citizen began with a tip from an informant. When the prosecutor pressed for more information, he said, a DEA supervisor intervened and revealed that the tip had actually come through the SOD and from an NSA intercept.
"I was pissed," the prosecutor said. "Lying about where the information came from is a bad start if you're trying to comply with the law because it can lead to all kinds of problems with discovery and candor to the court." The prosecutor never filed charges in the case because he lost confidence in the investigation, he said.
A senior DEA official said he was not aware of the case but said the agent should not have misled the prosecutor. How often such misdirection occurs is unknown, even to the government; the DEA official said the agency does not track what happens with tips after the SOD sends them to agents in the field.
The SOD's role providing information to agents isn't itself a secret. It is briefly mentioned by the DEA in budget documents, albeit without any reference to how that information is used or represented when cases go to court.
The DEA has long publicly touted the SOD's role in multi-jurisdictional and international investigations, connecting agents in separate cities who may be unwittingly investigating the same target and making sure undercover agents don't accidentally try to arrest each other.