The U.S. Postal Service has been photographing every package and letter mailed in the country, totaling billions of pieces of mail a year, for the last 17 years.
Former Postmaster General Patrick Donahoe told The Associated Press in 2013 that images of mail are taken at 200 processing plants around the U.S. and used to help sort mail. The process is called mail isolation and tracking system.
The automated mail tracking program was created after the deadly anthrax attacks in 2001 so the Postal Service could more easily track hazardous substances and keep people safe, Donahoe said. A week after the Sept. 11, 2001, terrorist attacks, letters laced with anthrax began arriving at media companies and lawmakers' offices; five people died as a result.
The images have also been used in the past to trace letters in criminal cases. The New York Times recently reported that law enforcement officials only need to fill out a form and submit it to the Postal Inspection Service in order to view mail.
It's unclear if the FBI used this system to track a dozen packages containing pipe bombs sent to elected officials and President Donald Trump critics in the last week. A man from Florida has been identified as a possible suspect, according to officials with the Department of Justice.
According to the USPS, the Postal Service photographs the front of letter-sized mail pieces that run through automation equipment.
The way Donahoe described the system in 2013, it could help the FBI track down the small packages, some were delivered by carrier, while others were mailed through USPS.
"We've got a process in place that pretty much outlines, in any specific facility, the path that mail goes through," Donahoe said. "So if anything ever happens, God forbid, we would be able very quickly to track back to see what building it was in, what machines it was on, that type of thing. That's the intent of the whole program."
USPS recently started using those images to provide digital notifications to customers with a new service called Informed Delivery. People receive an email with photos of their daily mail. Users can also view their mail on the USPS website for up to 15 days after it is delivered.
Some critics of the new service say it has opened them up to mail fraud and identity theft.