In his letter, Sanders cited the ruling last month by U.S. District Judge Richard J. Leon that the NSA's bulk collection of telephone metadata was probably unconstitutional and "almost Orwellian" in scope.

Later Friday, the Justice Department filed a one-page notice of appeal asking the U.S. Court of Appeals for the District of Columbia Circuit to overturn Leon's ruling. The appeal had been expected.

An aide to Sanders said the NSA had not responded to the senator's letter by late Friday.

Sanders has introduced legislation to limit the records the NSA and FBI may search and require officials to establish a reasonable suspicion, based on specific information, to secure court approval to monitor business records related to a specific terrorism suspect.

Some civil liberties experts who have been consulted by the White House are hoping Obama will make more sweeping changes than those his aides have disclosed so far.

"Public concern is not going to be allayed by occasional transparency or by more secret advocates in a secret court," said Michelle Richardson, legislative counsel for the American Civil Liberties Union, who has fielded questions from Obama's aides. "They want the government to stop spying on them. That's what we'll be looking for. Are we going to let the government spy on innocent people or not?"

Aides say Obama has emphasized that he doesn't want to restrain intelligence and law enforcement agencies to the point that they can't quickly investigate and interrupt terrorist activities.

James Lewis, senior fellow and director of the Strategic Technologies Program at the Center for Strategic and International Studies, cautioned against overlooking legitimate concerns of law enforcement and intelligence agencies. Al Qaeda, he said, is not a spent force.

"We overreacted after 9/11, and it makes sense to trim some of that back," he said. "But you don't want to trim it back so far that you increase the chances of another big event. That's the big concern."