As Smollett reported the attack to police, a multi-agency federal investigation into the source of the threat letter was entering its seventh day.

Headed by the FBI’s Chicago Field Office, the terroristic threat investigation is being assisted by one of America’s oldest and most underestimated law enforcement agencies: The United States Postal Inspection Service.

Whoever sent the letter to Smollett may have considered the mail to be an untraceable way to deliver a message. They’d be mistaken.

“Those postal guys are the real deal,” said a North Side cop who worked with inspectors on a serial package theft case in Lincoln Park last November. “They can do amazing things and aren’t afraid of the work.”

Since being created by Benjamin Franklin—yes, that Benjamin Franklin—the service has learned a few things about tracing mail-based crimes. Figuring out who mailed a letter has been made easier because, while most criminals know to wear gloves to avoid leaving fingerprints, most criminals also forget to leave their cellphones at home when they go to the mailbox.

Even without tracking phones, inspectors have a catalogue of techniques developed over the course of 200 years to track a letter to its source. More than 1,000 “white powder” cases were investigated between January 2007 and March 2009 alone, according the LA Times.