I have yet to hear a rational, truthful explanation from Bush administration officials to justify the domestic spying program. The claim that it was authorized by Congress post-9/11 falls apart with the most cursory examination. There are very specific laws in place around the authority of the government in these instances and those laws were passed specifically to curb abuses.

Alberto Gonzales gives assurances that it's all-so-legal but he lies:

The hearing got off to a testy start. Democrats wanted Gonzales to testify under oath, saying he had misled Congress in January when he dismissed a question about warrantless surveillance as "hypothetical."

An ACLU factsheet accompanying a story on their suit against the NSA for their domestic spying program includes this (emphasis mine):

By seriously compromising the free speech and privacy rights of the plaintiffs and all Americans, the ACLU charges that the NSA program violates the First and Fourth Amendments of the United States Constitution. The program authorizes the NSA to intercept the private communications of people who the government has no reason believe have committed or are planning to commit any crime, without first obtaining a warrant or any prior judicial approval.

The ACLU also charges that the program violates the constitutional principle of separation of powers, because it was authorized by President Bush in excess of his Executive authority and contrary to limits imposed by Congress. In response to widespread domestic surveillance abuses committed by the Executive Branch and exposed in the 1960s and 1970s, Congress enacted legislation that provides the exclusive means by which electronic surveillance and the interception of domestic wire, oral and electronic communications may be conducted. Congress enacted two statutes which impose strict limits on domestic surveillance, including prior judicial approval -- Title III of the Omnibus Crime Control and Safe Streets Act of 1968 and the Foreign Intelligence Surveillance Act (FISA), passed in 1978.

Congress enacted FISA after the U.S. Supreme Court held, in United States v. United States District Court for the Eastern District of Michigan, that the Fourth Amendment does not permit warrantless surveillance in intelligence investigations of domestic security threats. FISA amended Title III to provide that the procedures set out therein and in FISA "shall be the exclusive means by which electronic surveillance . . . and the interception of domestic wire, oral, and electronic communications may be conducted." FISA provides that no one may engage in electronic surveillance "except as authorized by statute," and it specifies civil and criminal penalties for electronic surveillance undertaken without statutory authority.

The legal reasoning from the Bush administration is so flimsy, it's apparent even to me that it's an illegal program.