Jake works for the NSA, and has operated within the U.S. (in mostly non signals-intelligence operations) in all but two episodes of the series shown in the UK thus far. The NSA is a signals intelligence agency that does not operate within the borders of the United States. They *could*, with a federal warrant, but why would they? It's not their job, so it's highly unlikely they would apply for such a warrant, and highly unlikely that one would be granted. There have been no indications that his operations are supposed to be illegal 'Black Ops'. Just about every mission Jake undertakes should be carried out by another agency; most of his domestic operations fall under the remit of the FBI, and most of the foreign ones would be the responsibility of the CIA. The NSA was probably chosen for the series due to its reputation as a mysterious organisation that uses esoteric technology, but it would have made more sense to create a fictitious organisation. [SIGINT is only a part of the NSA/CSS organization, the other parts of the organization have other functions internal to the US - not that what Jake does is not way beyond the usual parameters of the NSA, but it does fit that they "own" him because he is basically information technology. If a baker owns a hammer, you're not going to use it to make bread.]