The program's tool, RETRO, for “retrospective retrieval,” stores a bank of phone conversations; every 30 days, it clears out the oldest conversations to make way for new ones. The documents reveal the program is in use somewhere, but at the bequest of the NSA, the Post is withholding the country's identity for security reasons. The NSA documents outlined plans to add the operation in six other countries, also redacted by the Post. (Safe to assume there aren't too many countries out there who'd be happy about being part of this.) Only a small percentage of the calls are ever accessed, but there are apparently few limitations on the program: any calls from Americans caught in the foreign country's dragnet are also subject to collection.