CONNECT WITH:

By Justin Elliott, ProPublica 

Hijacker Khalid al Mihdhar, foreground, passes through security at Dulles International Airport hours before American Airlines Flight 77 slammed into the Pentagon

Hijacker Khalid al Mihdhar, foreground, passes through security at Dulles International Airport hours before American Airlines Flight 77 slammed into the Pentagon

In defending the NSA’s sweeping collection of Americans’ phone call records, Obama administration officials have repeatedly pointed out how it could have helped thwart the 9/11 attacks: If only the surveillance program been in place before Sept. 11, 2001, U.S. authorities would have been able to identify one of the future hijackers who was living in San Diego.

Last weekend, former Vice President Dick Cheney invoked the same argument.

By Justin Elliott and Theodoric Meyer, ProPublica 

National Security Agency headquarters in Fort Meade, Md.

National Security Agency headquarters in Fort Meade, Md.

Last week saw revelations that the FBI and the National Security Agency have been collecting Americans’ phone records en masse and that the agencies have access to data from nine tech companies.

But secrecy around the programs has meant even basic questions are still unanswered. Here’s what we still don’t know: