At 7:30 Sunday morning, when most Massachusetts residents were still sipping their coffee, Michael Chesna was doing his job. Thirty minutes before the scheduled end of his overnight shift, the 42-year-old Weymouth police officer, a father of two young children, responded to a call about an erratic driver in a residential area.

He ended up dead, shot in what officials describe as a cold-blooded killing.

If there were ever a reminder of the constant dangers that police officers face, the heinous killing surely is it. The state is united in mourning for Chesna, whom prosecutors say was shot by Emanuel Lopes, 20, the driver of the car, who managed to knock Chesna down with a rock, seize the officer’s weapon, then shoot him in the head and chest.

After shooting Chesna, officials say, Lopes also used the officer’s gun to kill a bystander, 77-year-old Vera Adams, who was on her sun porch. She appears to have been a completely random victim, a life snuffed out by stray bullets.

Soon after, Lopes was arrested; an arraignment is expected Tuesday on two counts of murder. Little is known about the suspect, but he had several run-ins with police, and had been arrested for selling cocaine to minors.

An outburst of mayhem on a quiet Sunday morning claimed two lives. It shattered countless others, including the friends and families of the victims, and the fellow officers who lined the streets for Chesna’s hearse on Sunday.

After such a senseless crime, there’s really no adequate response. In coming weeks, officials will need to scrutinize their past handling of Lopes to understand what, if anything, they could have done differently. But right now the focus is properly on coming together to show the community’s support for the families. The sacrifices made by officers like Michael Chesna can never be forgotten.