Friday's testimony by Defense Secretary Donald Rumsfeld and other top U.S. military officials before Congress is an example of the powers U.S. lawmakers have to summon executive branch officials for questioning.

They do so under the powers of congressional oversight, which allows the review, monitoring and supervision of federal agencies, programs, activities, and policy implementation.

These powers, derived from the U.S. Constitution and public law, allow Congress to summon key officials to testify on important issues, and to receive reports form executive branch officials on the activities of their organizations.

In addition, Congress members are responsible for approving the budget of federal agencies, including that of the United States armed services and the country's intelligence organizations.

Friday's hearings were held in part because Congress felt Mr. Rumsfeld had not sufficiently informed lawmakers about the investigations into the abuse of Iraqi prisoners by U.S. soldiers.