Suffolk County District Attorney Rachael Rollins said Monday that the Saturday killing of two Black residents of Winthrop — Ramona Cooper and David Green — should be a reminder that even largely liberal Massachussetts is not immune from hate-based killings.

On Saturday in Winthrop, 28-year-old Nathan Allen crashed a stolen truck into a building, exited the vehicle, and shot and killed Green, a retired Massachusetts State Police trooper, and Cooper, an Air Force veteran. Allen was subsequently shot and killed by police.

Rollins discussed the attack with Jim Braude on GBH's Greater Boston Monday.

“We believe he was motivated by hate,” Rollins said. Law enforcement, Rollins said, have found “hate-filled language and, sort of, rantings of a person that, in my opinion, is becoming or has already been radicalized.”

Investigators found writings in his handwriting that call the white race superior, as well as drawings of swastikas. In addition, after crashing the truck, Rollins said that he walked past several people that were not visibly people of color before shooting Green and Cooper.

The prosecutor added that the investigators believe he acted alone, though they have not yet gotten a full picture of what he’s read, websites he visited, or entities that he may have been affiliated with.

The shooting, Rollins noted, comes in context of other hate-motivated killings in the United States. “This is not something where our community, in Suffolk county — northerners as we are — to think, 'Oh, that doesn't happen here. That's just a southern problem,'” she said. “Or even, looking at the insurrection on January 6th, we believe, 'Oh, we don't know people like that, those are people that we read about.' This has now crept into our community.”

Rollins also noted that the shooter may not line up with many people's expectations due to “implicit bias.” He was married and had received a doctorate and, ahead of his wedding last year, was featured in a Boston Globe column about his fiancée and her grandfather. “This person had no criminal record, and went from zero entries on [his] criminal record to a double homicide — and many other potential crimes as well,” she said.

WATCH: Rollins spoke to Greater Boston's Jim Braude