WASHINGTON — The National Security Agency has been mining for several years its massive collections of email and phone call data to create extensive graphs of some Americans’ social connections that can include associates, travel companions and their locations, according to The New York Times.

The social graphing began in 2010 after the NSA lifted restrictions on the practice, according to an internal January 2011 memorandum, the Times reported online Saturday. It based its article on documents obtained by former NSA contractor Edward Snowden and interviews with officials.

The graphing, or contact chaining, is conducted using details about phone calls and emails, known as “metadata,” but does not involve the communications’ content, according to the documents cited by the Times. It is supposed to be done for foreign intelligence purposes only, the documents state, but that category is extremely broad and may include everything from data about terrorism and drug smuggling to foreign diplomats and economic talks.

The revelation is the latest in a string of disclosures that began in June, when The Washington Post and the British newspaper the Guardian broke stories, based on Snowden’s documents about the NSA’s PRISM program, which collects digital communications from U.S. Internet companies, and about the collection of call-detail records from U.S. phone companies.

Snowden’s disclosures and the subsequent declassification of records by Director of National Intelligence James Clapper Jr. and the nation’s secretive Foreign Intelligence Surveillance Court have sparked widespread concern over the scope of the NSA’s surveillance and whether it appropriately balances Americans’ privacy rights with national security.

The NSA did not provide an immediate response to the Times article.