The man who said on video he planned to assassinate Donald Trump at Friday's inauguration once spoke harshly about the 9/11 terrorists ... in no small part because his mom died on one of the planes.

Dominic Puopolo's mom, Sonia, was on American Airlines flight 11 from Boston to L.A. when it was hijacked and flown into the north tower of the World Trade Center.

The family made news back then because Sonia's hand with her wedding ring was found in the rubble.

Then-senator Hillary Clinton eulogized Sonia during her funeral in Boston. Clinton knew Sonia through work at the Red Cross. She was a big Hillary supporter.

After the attacks Dominic flew to Hamburg where he attended the trial of 9/11 mastermind Mounir El Motassadeq.