WASHINGTON, Dec. 20 - A surveillance program approved by President Bush to conduct eavesdropping without warrants has captured what are purely domestic communications in some cases, despite a requirement by the White House that one end of the intercepted conversations take place on foreign soil, officials say.
The officials say the National Security Agency's interception of a small number of communications between people within the United States was apparently accidental, and was caused by technical glitches at the National Security Agency in determining whether a communication was in fact "international."
Telecommunications experts say the issue points up troubling logistical questions about the program. At a time when communications networks are increasingly globalized, it is sometimes difficult even for the N.S.A. to determine whether someone is inside or outside the United States when making a cellphone call or sending an e-mail message. As a result, people that the security agency may think are outside the United States are actually on American soil.
Vice President Dick Cheney entered the debate over the legality of the program on Tuesday, casting the program as part of the administration's efforts to assert broader presidential powers.
Eavesdropping on communications between two people who are both inside the United States is prohibited under Mr. Bush's order allowing some domestic surveillance.
But in at least one instance, someone using an international cellphone was thought to be outside the United States when in fact both people in the conversation were in the country. Officials, who spoke on condition of anonymity because the program remains classified, would not discuss the number of accidental intercepts, but the total is thought to represent a very small fraction of the total number of wiretaps that Mr. Bush has authorized without getting warrants. In all, officials say the program has been used to eavesdrop on as many as 500 people at any one time, with the total number of people reaching perhaps into the thousands in the last three years.
Mr. Bush and his senior aides have emphasized since the disclosure of the program's existence last week that the president's executive order applied only to cases where one party on a call or e-mail message was outside the United States.
Gen. Michael V. Hayden, the former N.S.A. director who is now the second-ranking intelligence official in the country, was asked at a White House briefing this week whether there had been any "purely domestic" intercepts under the program.
"The authorization given to N.S.A. by the president requires that one end of these communications has to be outside the United States," General Hayden answered. "I can assure you, by the physics of the intercept, by how we actually conduct our activities, that one end of these communications are always outside the United States."
It ought to be rather easy to tell whether a call was inside or outside the US.
The plot thickens...