Donald Trump Jr. wasn't the only target of the man who allegedly mailed him a letter with suspicious white powder ... according to cops who busted the suspect in Massachusetts.
Daniel Frisiello was arrested and charged with mailing 5 threatening letters to high-profile individuals around the country ... including the federal prosecutor in Mark Salling's child porn case.
Frisiello's letter addressed to Nicola T. Hanna contained the white substance and included the following message -- "That’s for murdering Mark Salling! I Hope you end up the same place as Salling."
Similar letters were sent to Stanford Professor Michele Dauber, Michigan Senator Deborah Stabenow ... and Antonio Sabato Jr., who's running for a congressional seat in California.
As we reported ... Don Jr.'s wife Vanessa opened Frisiello's alleged letter intended for DTJ, exposing her to an unidentified white powder. It was later determined to be cornstarch.
Frisiello was arrested Thursday morning and is facing 5 counts of mailing a threat to injure, and 5 counts of false information and hoaxes.