Testifying in his own defense, Byrne admitted to less extensive physical contact with Trombly in the guard room but presented an alternate explanation, saying that he had rushed to restrain the student when he saw him reaching for a possible weapon in a deep denim pocket.

The sergeant said that Trombly’s sudden move had brought back memories of the 1993 death of academy classmate Thomas F. Rose, a BPD officer fatally shot with his own weapon by an arrestee in a station house as Byrne looked on.

Byrne said he had acted reflexively, filled with fear for his life, before realizing that Trombly was only grabbing for a cell phone.

But Trombly and the BPD officer who arrested him on Byrne’s orders testified that the student’s cell phone had in fact been confiscated long before, when he was frisked on Commonwealth Ave.

After the guard room altercation, Trombly was charged on counts of assault and battery on a police officer, resisting arrest and public drinking—all of which he said were wholly unfounded, and all of which were eventually thrown out by the Suffolk County District Attorney for lack of evidence.

In the aftermath of Trombly’s injuries, BPD witnesses told jurors of Byrne’s concerted efforts to keep them quiet about the abuse they’d seen as investigators turned up the heat.

Byrne maintained that he had only told them not to spread what he thought were untrue rumors—an argument which the jurors found unconvincing in each of the four cases.

In retrospect, Trombly said he thought the case against his attacker had been airtight—though he felt a conviction had been by no means certain.

“Any time you go up against the word of a police officer, you’d better have your facts straight and have a lot of evidence on your side,” he said.

—Staff writer Simon W. Vozick-Levinson can be reached at vozick@fas.harvard.edu.