Contents of the National Security Letter (NSL) have been revealed, showing how ISPs and telecommunications firms were asked to hand over customer data which includes sensitive information.

The information asked by the FBI through NSL includes complete web browsing history and records all online purchases, addresses associated with the account, subscriber telephone numbers, and all email addresses associated with account.

The letter was released by Nicholas Merrill, founder of Calyx Internet Access, who received the NSL in 2004 regarding one of his customers, following which he filed a First Amendment lawsuit in the same year.

Merrill refused to comply with the FBI’s NSL request, and eventually the FBI decided to drop its demand for records, but he was not allowed to speak in public.

However, a federal District Court judge in New York ruled in September that the ban on Merrill’s speech was not justified as the case was over and most of the details were available online. Following which Merrill released the details.

NSL is a law enforcement tool that has been used by US authorities since 1970s but their frequency increased after the 9/11 attacks when the USA Patriot Act came into picture.

Merrill also disclosed that the FBI may use NSLs to get IP addresses on everyone a suspect has corresponded with and cell-site location information.

However, the FBI claims that it no longer uses NSLs for location information.

The NSL has been haunting tech companies and privacy advocates for a long time who claim that the government snoops on user content without appropriate judicial oversight, reported Reuters.