I hear you. I’m guessing that someone tipped them off to his identity. He probably bragged that he had done it.
No, an off-duty state trooper noticed an SUV matching the description, and the driver at a gas station in Worcester. He wrote the plate number down on his hand. The trooper went to his apartment, and when no one answered, he left his business card with a note asking him to call. When he didn't call, the trooper paid him another visit, and asked if he would voluntarily give an saliva sample, which he did. The DNA matched material taken from under the victim's fingernails. Local and state policed arrested him.
Good old fashioned police work.