According to a Washington Post article, the FBI can issue a letter to an Internet Service Provider (ISP) or Financial Institution forcing them to hand over information on their customers. The Post article describes a situation where George Christian, who manages digital records for libraries in Connecticut, was approached by the FBI who demanded he turn over information about usage on a specific computer. They also warned him not to tell anyone about the demand, ever.

The Washington Post explains the nature of the letters:

The FBI now issues more than 30,000 national security letters a year, according to government sources, a hundredfold increase over historic norms. The letters — one of which can be used to sweep up the records of many people — are extending the bureau’s reach as never before into the telephone calls, correspondence and financial lives of ordinary Americans.

Issued by FBI field supervisors, national security letters do not need the imprimatur of a prosecutor, grand jury or judge. They receive no review after the fact by the Justice Department or Congress. The executive branch maintains only statistics, which are incomplete and confined to classified reports. The Bush administration defeated legislation and a lawsuit to require a public accounting, and has offered no example in which the use of a national security letter helped disrupt a terrorist plot.

The most disturbing part about this, to me at least, is the lack of checks and balances in place. This gives the FBI carte blanche to invade the privacy of any individual, at any time, for any reason, leaving individuals with little to no recourse.

Read more in the Washington Post article here.