WASHINGTON (Reuters) - FBI requests for customer records under a secretive surveillance order increased by nearly 50 percent in 2015, according to a U.S. government transparency report published this week.

A magnifying glass is held in front of a computer screen in this picture illustration taken in Berlin May 21, 2013. REUTERS/Pawel Kopczynski

Internet and telecommunications companies in 2015 received 48,642 requests, up from 33,024 reported in 2014, for data via so-called National Security Letters (NSLs). The NSL is a tool used by the Federal Bureau of Investigation to gather phone numbers, email and IP addresses, web browsing histories and other information.

An NSL does not require a warrant and is usually accompanied by a gag order.

The amount of actual written orders issued decreased in 2015, however, from 16,348 to 12,870. One NSL often contains multiple requests for information, such as a series of email addresses believed relevant to an investigation, where each address counts as one request.

The year-to-year statistics may not be entirely precise due to changes in reporting requirements ushered in last year under a surveillance reform law passed by Congress, sources familiar with the process said, but they indicate general trends.

The majority of NSL requests, 31,863, made in 2015 sought information on foreigners, regarding a total of 2,053 individuals, according to a Justice Department memo sent to Congress, while the amount of requests on U.S. persons declined.

A U.S. government source said the rise in NSL requests is in part attributable to efforts by militant groups such as Islamic State to use multiple accounts across several different communications platforms.

Islamic State, which has recruited known cyber experts from European countries and published treatises on communications security, is well known for using Twitter and other services to recruit and radicalize new adherents.

Andrew Crocker, a staff attorney with Electronic Frontier Foundation, a digital rights group, said the secretive nature of NSLs make it “impossible to know how substantial” government claims justifying their use are because they “aren’t tested in public.”

NSLs are a type of subpoena authority used to compel Internet and telecommunications companies to hand over customer data.

They are almost always accompanied by an open-ended gag order issued by the Justice Department barring companies from disclosing contents of the demand for customer data.

In March, the online forum Reddit removed a notice from its transparency report indicating it had begun receiving NSLs, prompting concern from its users and privacy advocates.

The FBI declined to comment Tuesday on the report.