The surveillance experts at the National Security Agency won't tell two powerful United States Senators how many Americans have had their communications picked up by the agency as part of its sweeping new counterterrorism powers. The reason: it would violate your privacy to say so.

That claim comes in a short letter sent Monday to civil libertarian Senators Ron Wyden and Mark Udall. The two members of the Senate's intelligence oversight committee asked the NSA a simple question last month: under the broad powers granted in 2008's expansion of the Foreign Intelligence Surveillance Act, how many persons inside the United States have been spied upon by the NSA?

The query bounced around the intelligence bureaucracy until it reached I. Charles McCullough, the Inspector General of the Office of the Director of National Intelligence, the nominal head of the 16 U.S. spy agencies. In a letter acquired by Danger Room, McCullough told the senators that the NSA inspector general "and NSA leadership agreed that an IG review of the sort suggested would itself violate the privacy of U.S. persons," McCullough wrote.

"All that Senator Udall and I are asking for is a ballpark estimate of how many Americans have been monitored under this law, and it is disappointing that the Inspectors General cannot provide it," Wyden told Danger Room on Monday. "If no one will even estimate how many Americans have had their communications collected under this law then it is all the more important that Congress act to close the 'back door searches' loophole, to keep the government from searching for Americans' phone calls and emails without a warrant."

What's more, McCullough argued, giving such a figure of how many Americans were spied on was "beyond the capacity" of the NSA's in-house watchdog – and to rectify it would require "imped[ing]" the very spy missions that concern Wyden and Udall. "I defer to [the NSA inspector general's] conclusion that obtaining such an estimate was beyond the capacity of his office and dedicating sufficient additional resources would likely impede the NSA's mission," McCullough wrote.

The changes to the Foreign Intelligence Surveillance Act in 2008 – which President Obama, then in the Senate, voted for – relaxed the standards under which communications with foreigners that passed through the United States could be collected by the spy agency. The NSA, for instance, no longer requires probable cause to intercept a person's phone calls, text messages or emails within the United States as long as one party to the communications is "reasonably" believed to be outside the United States.

The FISA Amendments Act of 2008, as it's known, legalized an expansive effort under the Bush administration that authorized NSA surveillance on persons inside the United States without a warrant in cases of suspicion of connections to terrorism. As my colleague David Kravets has reported, Wyden has attempted to slow a renewal of the 2008 surveillance authorities making its way through Congress. The House Judiciary Committee is expected to address the FISA Amendments Act on Tuesday, as the 2008 law expires this year.

Longtime intelligence watchers found the stonewalling of an "entirely legitimate oversight question" to be "disappointing and unsatisfactory," as Steve Aftergood, a secrecy expert at the Federation of American Scientists told Danger Room.

"If the FISA Amendments Act is not susceptible to oversight in this way," Aftergood said, "it should be repealed, not renewed."

Even though McCullough said the spy agencies wouldn't tell the senators how many Americans have been spied upon under the new authorities, he told them he "firmly believe[s] that oversight of intelligence collection is a proper function of an Inspector General. I will continue to work with you and the [Senate intelligence] Committee to identify ways we can enhance our ability to conduct effective oversight."