 -- The National Security Agency has begun "winding down" the once top-secret bulk collection of Americans' phone records after Congress failed to renew or change the program before a holiday recess, a senior administration official told ABC News.

"We've said for the past several days that the wind-down process would need to begin yesterday if there was no legislative agreement. That process has begun," the official said.

The government has relied on Section 215 of the Patriot Act, which expires at midnight on May 31, to authorize collection of telephone metadata for all U.S. calls. That data was said to include phone numbers and duration of a call, but not the content of the call or any other personally identifying information.

Former NSA contractor Edward Snowden revealed the existence of the phone surveillance program with his leaked cache of documents in 2013.

Lawmakers left town for a week-long holiday break without addressing the program as its expiration looms.