Unlike typical congressional hearings that feature testimony from various sides of a debate, the briefings in 2010 and 2011 on the telephone surveillance program were by definition one-sided affairs, with lawmakers hearing only from government officials steeped in the legal and national security arguments for aggressive spying.

Additional obstacles stemmed from the classified nature of documents, which lawmakers may read only in specific, secure offices; rules require them to leave their notes behind and restrict their ability to discuss the issues with colleagues, outside experts or their own staff.

While Senate Intelligence Committee members can each designate a full-time staffer for the committee who has full access, House members must rely on the existing committee staff, many of whom used to work for the spy agencies they are tasked with overseeing.

Agency officials, meantime, aggressively court the committee, giving lawmakers a sense of being insiders in a clandestine world and at times treating them to a real-life version of the Spy Museum, former members said.