The letters said the author sent information from American Airlines' safety and security manual to 25 Muslim charity organizations, the newspaper said, citing the criminal complaint in the case.
He also made threats against employees and enclosed graphic and violent photos, the newspaper reported.
According to the complaint, the airline fired Lorenzo after he threw a coffee pot at a co-worker during a flight.
The New York Post reported that Lorenzo was accused of threatening the co-worker after his dismissal.
Lorenzo has been charged with making terrorist threats and is free on a $50,000 bond.