The scheduled meeting in a secure room at the Capitol was attended by only 47 of the 100 senators, The Hill reported Saturday.
The Thursday afternoon briefing on classified surveillance programs included presentations by James Clapper, the director of national intelligence, and Keith Alexander, the head of the National Security Agency.
Sean Joyce, deputy director of the FBI, and the former chief judge of the Foreign Intelligence Surveillance Act court also spoke.
The low attendance frustrated Sen. Dianne Feinstein, D-Calif., the chair of the Senate Intelligence Committee.
"It's hard to get this story out," she said. "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."
Feinstein said she had spent the week answering questions about the NSA telephone-monitoring program, which had garnered numerous criticisms from her colleagues.
Many congressmen said they had never been briefed on the program when news of it broke in the British newspaper The Guardian.
"We have discussed this, we have voted on this in committee, on the floor," Feinstein noted. "People should go out and see how the program is set up, see how it's conducted, ask questions, come to the briefings."
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