It's Justice 101: There's the kind of evidence that you can use in court, and the kind you can't. When the two mix, it creates opportunities for the government to violate your right to mount an effective defense. Knowing this, the DEA has gone to great lengths to hide its use of non-admissible NSA data even as it leverages the information in the field.

There are a few reasons the DEA might want to cover up the NSA's involvement in criminal investigations. Not mentioning Fort Meade — or the DEA's Special Operations Division — preserves the secrecy that surrounds these organizations. It also superficially maintains the supposed wall that divides national security cases from other criminal investigations.

But prosecutors have a huge incentive not to reveal their surveillance assets for purely tactical reasons, too. Doing so could give defendants a make-or-break advantage, and a better chance at defending themselves in court.

It all comes down to a couple of Supreme Court cases that were decided decades ago. One isRoviaro v. United States, and it holds that the government must disclose the identity of an undercover witness if it will help the defendant prove he's not guilty.

The other case is Brady v. Maryland. That case established that if the prosecution deliberately withheld evidence that would help the defendant, it would amount to a violation of due process. The defendant could then ask the court to throw out the case.

What does that have to do with NSA surveillance? According to Edward Kang, a partner at Alston & Bird and a former Justice Department prosecutor, the security agency's intelligence isn't much different from the helpful testimony of a 1960s confidential informant. The government might argue that releasing the information would harm national security, but if a judge orders that it be handed over by reference to Roviaro or Brady, the prosecution would have to comply.

"If the government fails to make that disclosure," said Kang, "the defense could potentially move to dismiss the indictment either for failing to turn over exculpatory material under Brady — because one of the remedies that a court can consider for Brady violations is dismissal of the indictment — or due process under a theory that the government has exhibited outrageous conduct."

If you're wondering, "outrageous government conduct" is an actual legal accusation. If you can prove it, it can be enough to let you off the hook.

The intelligence handed over might not be enough to acquit a defendant alone. But it opens up more tactical possibilities for the defense, which may partly explain why the DEA went to such lengths to cover up its use of NSA resources.

It gets worse for the government. When law enforcement asks for permission to conduct a wiretap or other surveillance, it has to get court approval. (In the NSA's case, the body in question is the super-secretive Foreign Intelligence Surveillance Court.) In doing so, the authorities have to describe the details of the case to the judge. But if the DEA is hiding the source of its investigative leads from the public, said Kang, are they also hiding it from the judges whose job it is to approve further surveillance?

"If it's done knowingly and intentionally, then that's a criminal offense for making a false statement to a judicial officer," Kang said. "That could be a separately punishable felony offense, or viewed in the context of civil or criminal contempt."

It may be tough for a defendant to know whether his prosecutors were relying on secret NSA intelligence in the first place. But if the fact comes out, it can turn into a big headache for prosecutors.