The Post said the MYSTIC program developed in 2009 allows the agency to retrieve and listen to calls for a month after they were made. A classified summary the newspaper obtained said the program includes a tool known as RETRO for retrospective retrieval that allows calls to be stored for 30 days.
Monitors can "retrieve audio of interest that was not tasked at the time of the original call," the summary said. While that amounts to a tiny fraction of the calls made within the country it adds up to millions of pieces of audio.
Caitlin Hayden, a spokeswoman for the National Security Council, would not comment on any specific intelligence messages. She said generally new threats are "often hidden within the large and complex system of modern global communications, and the United States must consequently collect signals intelligence in bulk in certain circumstances in order to identify these threats."
The Post said the RETRO tool was built for use against a specific target country. But the secret intelligence budget adopted last year names at least five more potential targets.