Congress Wasn’t Even Informed About It
Constitutional law expert Bruce Fein and well-known author and NSA expert James Bamford point out that the NSA is the only Federal agency which was created without Congressional authorization or debate.
Indeed, the NSA was created by President Truman in 1952 through a secret memo, and Congress was kept in the dark:
Correction: Matt Stoller notes that the FBI was not created by Congressional legislation either, pointing us to Wikipedia:
The Bureau of Investigation (BOI) was created on July 26, 1908, after Congress had adjourned for the summer. Attorney General Bonaparte, using Department of Justice expense funds, hired thirty-four people, including some veterans of the Secret Service, to work for a new investigative agency. Its first chief (the title is now known as director) was Stanley Finch. Bonaparte notified Congress of these actions in December, 1908.