https://www.scientificamerican.com/article/how-do-bomb-squads-assess-a-suspicious-package/

Since Monday at least 12 potentially explosive devices have been found in or intercepted on their way to the mailboxes of Democratic political leaders, prominent public figures and CNN’s New York City office. Authorities have now arrested Cesar Sayoc, 56, of Aventura, Fla., in connection with the bombs, according to news reports.

The packages found so far—whose intended recipients include former Pres. Barack Obama, presidential candidate Hillary Clinton and, most recently, New Jersey Sen. Cory Booker and former Director of National Intelligence James Clapper—looked dubious enough to be caught before they could cause harm. But they were just tape-bound, bubble wrap–lined manila envelopes. How do law enforcement officials determine whether a suspicious mailed package is a real threat?