After a contentious three-year debate that was set off by the disclosure in 2005 of the program of wiretapping without warrants that President George W. Bush approved after the Sept. 11 attacks, Congress gave the N.S.A. broad new authority to collect, without court-approved warrants, vast streams of international phone and e-mail traffic as it passed through American telecommunications gateways. The targets of the eavesdropping had to be “reasonably believed” to be outside the United States. Under the new legislation, however, the N.S.A. still needed court approval to monitor the purely domestic communications of Americans who came under suspicion.

In recent weeks, the eavesdropping agency notified members of the Congressional intelligence committees that it had encountered operational and legal problems in complying with the new wiretapping law, Congressional officials said.

Officials would not discuss details of the overcollection problem because it involves classified intelligence-gathering techniques. But the issue appears focused in part on technical problems in the N.S.A.’s ability at times to distinguish between communications inside the United States and those overseas as it uses its access to American telecommunications companies’ fiber-optic lines and its own spy satellites to intercept millions of calls and e-mail messages.

One official said that led the agency to inadvertently “target” groups of Americans and collect their domestic communications without proper court authority. Officials are still trying to determine how many violations may have occurred.

The overcollection problems appear to have been uncovered as part of a twice-annual certification that the Justice Department and the director of national intelligence are required to give to the Foreign Intelligence Surveillance Court on the protocols that the N.S.A. is using in wiretapping. That review, officials said, began in the waning days of the Bush administration and was continued by the Obama administration. It led intelligence officials to realize that the N.S.A. was improperly capturing information involving significant amounts of American traffic.

Notified of the problems by the N.S.A., officials with both the House and Senate intelligence committees said they had concerns that the agency had ignored civil liberties safeguards built into last year’s wiretapping law. “We have received notice of a serious issue involving the N.S.A., and we’ve begun inquiries into it,” a Congressional staff member said.

Separate from the new inquiries, the Justice Department has for more than two years been investigating aspects of the N.S.A.’s wiretapping program.