According to the a DOJ investigation, the FBI has violated the law on several occasions in connection with the issuance of National Security Letters (NSLs). A NSL is a demand letter issued to a particular entity or organization to turn over various record and data pertaining to individuals. They do not require probable cause, a warrant, or even judicial oversight. They also come with a gag order, preventing the recipient of the letter from disclosing that the letter was ever issued. Compliance is mandatory.
From the New York Times:
The Pentagon and to a lesser extent the CIA have been using a little-known power to look at the banking and credit records of hundreds of Americans and others suspected of terrorism or espionage within the United States, officials said Saturday.
The C.I.A. has also been issuing what are known as national security letters to gain access to financial records from American companies, though it has done so only rarely, intelligence officials say.
A recent article in the LA Times discusses the ongoing debate about whether to remove J. Edgar Hoover’s name from the FBI building. J. Edgar Hoover was the head of the FBI from 1924, its early days before it was even called the FBI (it used to be called the Bureau of Investigation), until his death in 1972. Throughout his career, Hoover engaged in a massive array of abuses. According to the LA Times article:
Did you know that the FBI can issue a letter to an Internet Service Provider or a financial institution demanding that they turn over data on a customer? The letter doesn’t require probable cause. No judge must authorize the letter. The FBI simply issues the letter and gets the information. There’s a gag order, too, preventing the institution receiving the letter from mentioning this fact.