As anger continues to foment over the US National Security Agency's controversial surveillance programs, a confidential briefing on the topic held Thursday was attended by only 47 of 100 US senators.

The Hill reports that the briefing hosted several key players in the formation and execution of government surveillance programs, including NSA director Gen. Keith Alexander and Director of National Intelligence James Clapper, as well as a former judge from the secret FISA court which authorizes surveillance requests. But many senators instead elected to catch early flights home for Father's Day weekend, leaving dozens of chairs in the chamber empty.

The meeting comes a little more than a week after a series of disclosures published by The Guardian and the Washington Post showed that the NSA has been collecting the phone records and metadata of all Verizon business customers on an "ongoing, daily basis," and obtaining private communications from services such as Facebook, Google, and Skype under a program called PRISM. The Hill was not able to learn who did and did not attend the session.

"It’s hard to get this story out. Even now we have this big briefing — we’ve got Alexander, we’ve got the FBI, we’ve got the Justice Department, we have the FISA Court there, we have Clapper there — and people are leaving," said Senate Intelligence Committee chair Diane Feinstein. "If members were more diligent about attending briefings they would be far better informed about what’s going on, and they would also be far more willing to challenge the intelligence community on the conclusions that they come to." NSA chief Gen. Keith Alexander has claimed that the surveillance programs have thwarted "dozens" of terrorist attacks, and directed the agency to declassify the exact number by sometime next week.

Both Feinstein and Senate Majority Leader Harry Reid have adamantly defended the programs, and repeatedly insisted that members of Congress had been fully briefed. But many members have said otherwise, suggesting the true nature of the programs may have been concealed.

Congress has voted twice for the FISA Amendments Act, which extends government surveillance powers. But secret interpretations of section 215 of the Patriot Act and section 702 of the FISA Amendments Act — which authorize metadata collection and PRISM internet surveillance, respectively — have left the majority of Congress in the dark about how the law is actually applied. NPR is reporting that the Obama administration is considering declassifying a key court order that authorized the NSA's collection of phone records and metadata.