When ricin-laced letters were sent to Mayor Michael Bloomberg and President Barack Obama, the FBI used a secret program to track down the sender. It's called the Mail Isolation Control and Tracking program. Through it, U.S. Postal Service computers photograph the exterior of every piece of paper mail - last year, it scanned 160 billion pieces.
WNYC's Amy Eddings interviewed New York Times reporter Ron Nixon, who brought attention to this story.