The Mad Bomber: George Metesky

For 16 years, spanning from the 1940s to 1950s, bombs were left around prominent locations of New York City, including Consolidated Edison buildings. The only information surrounding who might be responsible for these attacks came down to notes sent and left by the bomber signed, "F.P." That's when investigators had a novel idea: talk to someone trained in psychology and come up with a sort of 'picture' of who might carry out this type of terrorism...and it worked! 

Sources:

https://en.wikipedia.org/wiki/George_Metesky

https://www.loc.gov/pictures/item/2016649583/

https://www.apa.org/monitor/julaug04/criminal#:~:text=In%201956%2C%20the%20frustrated%20investigators,and%20notes%20from%20the%20bomber.