President Bush has revealed he authorised a US intelligence agency to eavesdrop within the United States without court approval. The BBC News website considers some key questions:
1. What did the president order?
"In the weeks following the terrorist attacks on our nation, I authorised the National Security Agency, consistent with US law and the Constitution, to intercept the international communications of people with known links to al-Qaeda and related terrorist organisations," Mr Bush said on 17 December.
His announcement followed a New York Times report that the president had allowed US intelligence agencies to monitor phone calls and e-mail sent by US citizens without getting permission from the courts.
2. Is this legal?
It is difficult to say.
Civil liberties groups and some senators are concerned about a possible breach of the Fourth Amendment to the US Constitution.
This protects the "right of people to be secure in their persons, houses, papers and effects, against unreasonable searches and seizures". Electronic surveillance did not exist in 1789 when this was written, but over the years the limits of the government's powers in relation to new technologies have become more clearly defined.