Google has published eight secret requests from the Federal Bureau of Investigation which sought customer data between 2010 and 2015.

Two months after being freed from a gag order, the internet company has released redacted versions of the subpoena letter and seven others. It has also published letters with the FBI relating to the release of the documents

"In our continued effort to increase transparency around government demands for user data, today we begin to make available to the public the National Security Letters (NSLs) we have received where, either through litigation or legislation, we have been freed of nondisclosure obligations," Richard Salgado, Google’s director of law enforcement and information security policy, wrote in a statement.

Google and major tech companies still face several hundred thousand subpoenas every year and most of them are not allowed to be released. The US government made increasing requests after the introduction of the US Patriot Act following the 9/11 terrorist attacks.

The eight documents give an indication of the type of information requested by the government, and how internet companies are fighting those requests in court. One recent high profile example was when the FBI requested Apple to unlock the phone of a suspected terrorist in San Bernardino, California, and then got hold of the information without Apple’s help.

The move from Google follows Yahoo publishing three letters in June – the first internet company to do so – which revealed the intelligence agency had asked for email and browsing records of customers – more information than it was legally allowed to ask for.

The requests from the FBI are called National Security Letters, which are used to force third parties to hand over information pertaining to a government investigation. They are legal documents but are not signed by a judge.

One document from an agent in North Carolina in September 2014 asked Google for names, addresses and account information for 11 Gmail accounts. The details were redacted in the released document, as well as the agent asking for it to ensure their "safety".

Until the USA Freedom Act was passed in June 2015, agencies issuing NSLs could impose infinite gag orders.

Following years of controversy surrounding FBI’s NSLs, the FBI must now review the gag order either three years after the date it was sent, or at the end of the agency's investigation.

"In the near future, we will establish a more permanent home for these and additional materials from our Transparency Report," Mr Salgado wrote.

Last week the FBI was reportedly forced to re-issue thousands of NSLs as it had failed to inform recipient firms that the law around challenging them had changed. Tech firms could now make multiple appeals per year, as opposed to just one.