Shortly after the September 2001 terrorist attacks, the NSA began collecting American phone metadata. The program came to light when former agency contractor Edward Snowden revealed in 2013 that millions of American's calling records had been collected.

President Barack Obama sought, and Congress passed, a law ending the collection and instead allowing the NSA to request the records from phone companies as needed in terrorism investigations. The USA Freedom Act signed by the President on June 2, prohibits the bulk collection of Americans' telephone records and instead requires intelligence agencies to obtain targeted records directly from telephone companies, according to Wall Street Journal.

The law has given the NSA six months to wind down an existing program that swept up almost all records from phone companies and stored the information - including phone numbers called by individuals and the duration of conversations - on government servers.

The government announced on Monday that The National Security Agency will purge all phone data collected during the operation of its expiring bulk surveillance program by the start of next year pending ongoing litigation. "As soon as possible, NSA will destroy the Section 215 bulk telephony metadata upon expiration of its litigation preservation obligations," said a statement by the Office of the Director of National Intelligence. "Analytic access" to those records, which go back five years, will end Nov. 29, and they will be destroyed three months later.

Privacy advocates, who worried that the winding down of the mass surveillance program under a reform law enacted earlier this year could have allowed the NSA to continue to access phone records it already had collected, were jubilant after the decision, reported the National Journal.

Giving his views on the recent development, Steven Aftergood, intelligence and secrecy specialist for the Federation of American Scientists said: "There's a potential reduction in capability that they are accepting under pressure. Whatever intelligence and analytical value might reside in this data will be eliminated.

"It's a political choice that they are making, and it shows that at the end of the day they are a law-abiding organization. They are not putting their intelligence interests above external control," he said, according to The Guardian.