Karen Read’s second murder trial began Tuesday with a prosecutor saying the defendant’s own words will bolster evidence that she killed her police officer boyfriend three years ago and a defense attorney calling the case “the definition of reasonable doubt.

The second murder trial of Karen Read, whose case has sparked a national debate on police accountability and won her legions of supporters, began Tuesday, with prosecutors and defense lawyers presenting theories during their opening statements about how her Boston police officer boyfriend ended up dead.

An accused killer was arrested late Monday night after police said he took off from an apartment in Boston, where a woman was found dead.

The 26-year-old man accused of killing a woman inside an Allston apartment on Monday night was ordered to undergo a mental competency evaluation.

Omar Riaz, 26, of Allston, was arrested after officers responded to a radio call for a possible domestic violence incident at 82 Glenville Ave., police said.