U.S. Postal Inspectors: Mail Bombs
https://postalinspectors.uspis.gov/radDocs/bombs.htm
U.S. Postal Inspectors: History of Mail Bombs
https://postalmuseum.si.edu/behindthebadge/bombs.html
USPS: Protect Your Business from Package Bombs and Bomb Threats
http://about.usps.com/publications/pub166/pub166fm_012.htm
USPS: Letter or Package Bombs and Bomb Threats
https://about.usps.com/publications/pub166/pub166_tech_013.htm
USPS: Handling and Processing Mail Safely
https://about.usps.com/publications/pub166/pub166_tech_015.htm
USPS: Suspicious Mail or Packages poster
https://about.usps.com/posters/pos84.pdf
“The Ease of The Postal Service Makes It a Vector for Violence: A short history of how the mail has been exploited for terror,” Sarah Zhang, The Atlantic, March 13, 2018
https://www.theatlantic.com/technology/archive/2018/03/mail-postal-service-bombs/555440/
“Package Bombs Are Rare in the U.S., But Here’s How to Spot One,” John Donovan, Science: How Stuff Works, March 13, 2018
https://science.howstuffworks.com/package-bombs-are-rare-in-us-how-to-spot-one.htm
“Letter Bomb,” Wikipedia
https://en.wikipedia.org/wiki/Letter_bomb