The Guardian reports: Two prominent Senate critics of the NSA’s dragnet surveillance have challenged the agency’s assertion that the spy efforts helped stop “dozens” of terror attacks.

Mark Udall and Ron Wyden, both members of the Senate intelligence committee, said they were not convinced by the testimony of the NSA director, General Keith Alexander, on Capitol Hill on Wednesday, who claimed that evidence gleaned from surveillance helped thwart attacks in the US.

“We have not yet seen any evidence showing that the NSA’s dragnet collection of Americans’ phone records has produced any uniquely valuable intelligence,” they said in a statement released on Thursday ahead of a widely anticipated briefing for US senators about the National Security Agency’s activities.

“When you’re talking about important liberties that the American people feel strongly about, and you want to have an intelligence program, you’ve got to make a case for why it provides unique value to the [intelligence] community atop what they can already have,” Wyden, an Oregon Democrat, told the Guardian in an interview on Thursday.