The New York Times

Under a presidential order signed in 2002, the intelligence agency has monitored the international telephone calls and international e-mail messages of hundreds, perhaps thousands, of people inside the United States without warrants over the past three years in an effort to track possible "dirty numbers" linked to Al Qaeda, the officials said. The agency, they said, still seeks warrants to monitor entirely domestic communications.

The previously undisclosed decision to permit some eavesdropping inside the country without court approval was a major shift in American intelligence-gathering practices, particularly for the National Security Agency, whose mission is to spy on communications abroad. As a result, some officials familiar with the continuing operation have questioned whether the surveillance has stretched, if not crossed, constitutional limits on legal searches.

"This is really a sea change," said a former senior official who specializes in national security law. "It's almost a mainstay of this country that the N.S.A. only does foreign searches."

Nearly a dozen current and former officials, who were granted anonymity because of the classified nature of the program, discussed it with reporters for The New York Times because of their concerns about the operation's legality and oversight.