This was the key question, the one that she dreaded answering, because of the consequences that would follow.

“Homicide,” said Maura. It was not her job to point out the guilty party. She restricted her answer to that one word, but she couldn’t help glancing at Wayne Graff. The accused police officer sat motionless, his face as unreadable as granite. For more than a decade, he had served the city of Boston with distinction. A dozen character witnesses had stepped forward to tell the court how Officer Graff had courageously come to their aid. He was a hero, they said, and Maura believed them.

But on the night of October 31, the night that Fabian Dixon murdered a police officer, Wayne Graff and his partner had transformed into angels of vengeance. They’d made the arrest, and Dixon was in their custody when he died. Subject was agitated and violent, as if under the influence of PCP or crack, they wrote in their statement. They described Dixon’s crazed resistance, his superhuman strength. It had taken both officers to wrestle the prisoner into the cruiser. Controlling him required force, but he did not seem to notice pain. During this struggle, he was making grunts and animal sounds and trying to take off his clothes, even though it was forty degrees that night. They had described, almost too perfectly, the known medical condition of excited delirium, which had killed other cocaine-addled prisoners.

But months later, the toxicology report showed only alcohol in Dixon’s system. It left no doubt in Maura’s mind that the manner of death was homicide. And one of the killers now sat at the defense table, staring at Maura.

“I have no further questions,” said Aguilar. She sat down, looking confident that she had successfully made her case.

Morris Whaley, the defense attorney, rose for the cross-examination, and Maura felt her muscles tense. Whaley appeared cordial enough as he approached the witness stand, as if he intended only to have a friendly chat. Had they met at a cocktail party, she might have found him pleasant company, an attractive enough man in his Brooks Brothers suit.