The National Security Agency's independent watchdog reviewed its controversial domestic surveillance program over several years, a longer period than previously known, according to an agency official.

The inspector general, Joel Brenner, has looked into all aspects of the program's operations to ensure they complied with orders signed by President Bush, according to an NSA official familiar with the inspector general. The official spoke on condition of anonymity Thursday because of the sensitive nature of the program.

Bush has acknowledged he authorized the NSA to eavesdrop — without warrants — on international calls and e-mails of Americans and others inside the United States with suspected ties to Al Qaeda or its affiliates.

Last month, the country's No. 2 intelligence official, Gen. Michael Hayden, said the activities "have intense oversight" by the NSA's inspectors general and Justice Department officials. Hayden headed the NSA when the program was launched after Sept. 11, 2001.

Yet many details of the program remain unknown. Before Thursday, basic questions about the nature of the inspector general's review were unclear.

The NSA official said the inspector general's work is not in response to a specific incident. Rather, the review spans several years and included audits of actions and processes set up as part of the monitoring.