National Security Agency director Gen. Keith Alexander faced a skeptical crowd at the annual Black Hat hackers convention in Las Vegas on Wednesday, even before news broke that NSA leaker Edward Snowden had been granted temporary asylum in Russia.

Trust me, Mr. Alexander told the group of corporate cybersecurity specialists and hackers: Reports of NSA analysts roaming unsupervised through the private communications of millions of Americans are greatly exaggerated. The agency, he insisted, goes to great lengths to ensure that every search conducted by its employees is properly authorized and monitored.

Except, apparently, those conducted by Mr. Snowden — and who knows how many others like him. Mr. Snowden had already claimed he could gain access to any piece of information he wanted as a systems administrator of the massive databases of telephone and email communications compiled by the agency. It's no wonder members of the audience openly questioned whether anything the NSA director said could be believed.

That, in a nutshell, is the whole problem the NSA is now facing as a result of Mr. Snowden's revelations. The agency operates in secrecy, and its activities are authorized by a secret court whose legal reasoning and rulings are also secret. Even the congressional committees that are supposed to oversee it meet in secret sessions closed to the public. A heckler asked Mr. Alexander why we should believe him, and the answer is that we can't because there's no way of knowing what the NSA is actually doing.

That's also why Mr. Snowden's leak and subsequent flight to Russia are so hard to countenance, even by people who are troubled by the extent of the NSA surveillance program he revealed. It's one thing to alert Americans that their rights may have been violated by government snooping. It's quite another to in effect hand-deliver four computers worth of classified government information to a foreign power that means us no good.

It's inconceivable that Mr. Snowden didn't know this when he fled first to China and then Russia after leaking details of NSA surveillance programs to The Guardian, one of Great Britain's largest newspapers. And it was pure hubris on his part if he believed those countries didn't have the technical capability to gain access to data in the computers he took with him, with or without his permission. It's likely Russia's grant of temporary asylum is just a way of keeping the U.S. guessing a little longer about what secrets it has already pilfered from the fugitive NSA contractor.

In any case, for the foreseeable future, Mr. Snowden is apparently beyond the reach of U.S. authorities who wish to prosecute him. It remains to be seen what impact his actions will have on U.S.-Russia relations. Reports that President Barack Obama has considered canceling a planned meeting with Russian President Vladimir Putin suggest the depth of U.S. displeasure with the Kremlin's decision to grant Mr. Snowden asylum. But that might be simply closing the barn door after the horses have escaped if Russia's security services are already in possession of the information in Mr. Snowden's computers.

General Alexander said at a congressional hearing last month that the NSA's surveillance program had thwarted more than 50 "terrorist events" since the Sept. 11, 2001 attacks, including at least 10 "homeland-based threats." But when pressed Wednesday, an NSA official testified before a Senate panel that there was only one case that comes close to an example of a terrorist attack that would have occurred if not for the telephone surveillance program.

The NSA's evolving answers and selective disclosures are feeding a growing sense in Congress that the agency is operating without sufficient oversight or control. That view was evident last week, when libertarian Republican conservatives and progressive Democrats joined forces behind a measure to defund the NSA's surveillance activities that fell just seven votes short of passage. The Obama administration needs to offer more than vague promises of reform, and if it doesn't, Congress must step in and impose real limits and oversight.