The U.S. Postal Service reminds local employees and customers to be safe, watchful and observant regarding the proper identification and disposition of suspicious packages.

The warning comes on the heels of more than a dozen pipe bombs that were sent to prominent Democrats, public figures and CNN.

Authorities believe the packages, which were intercepted before reaching their intended recipients, all went through the U.S. Postal Service at some point, a source said. None detonated and no one has been hurt.

Mail bombs are rare, the Postal Inspection Service said on its website. The service investigates 16 mail bombs on average annually while processing 170 billion pieces of mail.

Liz Davis, public information officer for Postal Inspector Phoenix Division, said the Postal Service utilizes a comprehensive approach of specialized technology, screening protocols and employee training. Included are portable x-ray machines, but details were not revealed because of security reasons.

“In order to prevent attempts to compromise the effectiveness of our investigative methods, the U.S. Postal Inspection Service does not comment publicly on our investigative procedures and operational protocols,” Davis said.

Suspicious clues are misspelled words, incorrect titles or addressed to a position only, excessive tape and no return address.

If a package or mail is suspect, call 911, isolate the area immediately and wash hands with soap and water.