Two U.S. senators challenged the National Security Agency over its explanation of the legal authority behind its broad surveillance program, alleging that a fact sheet put out by the agency contained a "significant" inaccuracy. 

Sens. Mark Udall, D-Colo., and Ron Wyden, D-Ore., made the allegation in a letter to NSA Director Gen. Keith Alexander. They strained to explain the alleged inaccuracy, though -- as the underlying information was classified, they could not include any specifics in their public letter. 

They would only say, "in our judgment this inaccuracy is significant, as it portrays protections for Americans' privacy as being significantly stronger than they actually are." 

The senators urged Alexander to "correct this statement as soon as possible." 

Alexander and other intelligence officials will brief House lawmakers on surveillance activities Wednesday afternoon.

The senators were referring broadly to the fact sheet put out by the agency on Section 702 of the Foreign Intelligence Surveillance Act. The fact sheet said the surveillance authority extends only to the targeting of "foreign persons" located outside the U.S. It also said any communication "inadvertently acquired" on a U.S. person has to be destroyed if it is not relevant or evidence of a crime. 

On the latter claim, Udall and Wyden said the statement was "somewhat misleading," as it implies the NSA can determine how many "American communications" it has collected. In fact, the senators wrote, they've been assured the NSA does not have that ability. 

Asked for comment, NSA spokeswoman Judith Emmel referred back to the law. 

"Given the intense interest from the media, the public, and Congress, we believe the precision of the source document (the statute) is the best possible representation of applicable authorities," she said.