If a person in government says the sun will come up tomorrow, it's sensible to believe that person—but not until the first rays seep over the horizon. Skepticism is even more justified when the government has been caught hiding something from the public and needs to excuse the secrecy.

In the aftermath of Edward Snowden's leaks about surveillance conducted by the National Security Agency, its defenders have made two basic assertions. The first is that these programs were vital in stopping terrorists. The second is that by revealing their mere existence, Snowden did grave damage to national security.

On the first claim, NSA Director Keith Alexander and others have gone into great detail, crediting the programs for foiling some 50 plots. But on the second one, they have been curiously reticent.

Now, it's hard to believe it would come as a great surprise to Al Qaeda that American spies might be examining their phone records. Nor is it likely that hardened militants were slapping their foreheads to learn that someone in Washington may have been reading their email or listening in on their Skype chats.

But those in power insist that unveiling the information put lives at risk. Rep. Mike Rogers, R-Mich., chairman of the House Intelligence Committee, decried these "dangerous national security leaks," insisting that the "effectiveness of these programs depends on them being kept secret from the foreign terrorists they target."

Alexander echoed that claim in testifying before the committee last week. Asked whether the revelation was harmful to security, he replied, "I think it was irreversible and significant damage to this nation. … I believe it will hurt us and our allies."

But how? My curiosity whetted, I contacted Rogers' office for information on what the terrorists gained. A spokesman emailed to say the chairman could not be bothered to offer support for his allegation: "He does not have space available in his schedule this week to re-address issues that have very clearly been addressed in the open hearing." No transcript of the hearing was available, but I was assured I would get the answers if I watched the video.

Filled with hope, I watched all three hours—but was disappointed. Only three times did the subject come up at all, and then briefly. No one offered anything to substantiate the claim.

I continued to pester for evidence that would vindicate Rogers. At length, I got a statement from the chairman, which said, "Providing details of how terrorists have adapted their behavior based on what they have learned from the recent leaks would only compound the damage those leaks have done by giving our adversaries feedback on the quality of their work. I will say, broadly, that these leaks have done serious damage to our ability to detect and disrupt terrorist plots."

That evasion would be more believable if it hadn't occurred after officials spent hours publicly detailing how the NSA and FBI operate. Or if those officials had not shined a floodlight on specific plots and how these programs pulverized them.

It shouldn't be impossible to give Americans some sense of how the disclosures will help the terrorists. Will they get rid of their cellphones? Stay off the Internet? Use smoke signals? Actually, those don't sound like harms from the leaks—they sound like benefits.

Maybe there are other obvious things they could do. Rogers avows he would be aiding our enemies if he said a word about what they might do. But our enemies already know what they can do. The only people who don't are ordinary Americans.

I hoped to have better luck with Dianne Feinstein, D-Calif., head of the Senate Intelligence Committee, who says Snowden committed "treason." I called her press secretary, asked the same question I had asked Rogers' spokesperson and was promised a response. Despite additional calls, I never got it.

FBI Director Robert Mueller, appearing before the Senate Judiciary Committee on Wednesday, finally offered something tangible on the point. The leaks endanger Americans, he explained, because "you have persons who want to undertake terrorist attacks who don't have a full understanding of the Internet. And, to the extent that you expose programs like this, we are educating them."

Yes, he really said that. Maybe Mueller thinks the terrorists are incredibly dumb. Or maybe he just thinks we are.