The Federal Bureau of Investigation has used a secretive authority to compel Internet and telecommunications firms to hand over customer data including an individual's complete web browsing history and records of all online purchases, a court filing released Monday shows. The documents are believed to be the first time the government has provided details of its so-called national security letters, which are used by the FBI to conduct electronic surveillance without the need for court approval. National security letters have been available as a law enforcement tool since the 1970s, but their frequency and breadth expanded dramatically under the USA Patriot Act, which was passed shortly after the Sept. 11, 2001 attacks. They are almost always accompanied by an open-ended gag order barring companies from disclosing the contents of the demand for customer data. The secretive orders have long drawn the ire of tech companies and privacy advocates, who argue NSLs allow the government to snoop on user content without appropriate judicial oversight. Last year, the Obama administration announced it would permit Internet companies to disclose more about the number of NSLs they receive. But they can still only provide a range such as between 0 and 999 requests. Twitter has sued in federal court seeking the ability to publish more details in its semi-annual transparency reports. Several thousand NSLs are now issued by the FBI every year. At one point that number eclipsed 50,000 letters annually.