WATERTOWN — A sleepy Sunday morning erupted into a nightmare after three people — including an elderly woman — were found shot to death inside a Boyd Street home, transforming a quiet neighborhood into the scene of a triple killing.

Police say they arrived around 10 a.m. for a well-being check. Instead, officers stepped into bloodshed: an elderly resident dead on one floor, and a man and woman — who lived in a separate part of the same unit — lying lifeless elsewhere inside.

By mid-morning, Boyd Street looked like something out of a crime drama: cruisers blocking off the block, detectives hauling in evidence bags, neighbors whispering in shock as yellow tape wrapped the home now branded a house of horror.

Middlesex District Attorney Marian Ryan and Watertown Police Chief Justin Hanrahan said the victims knew each other and stressed that the massacre appears “isolated,” with no ongoing threat to the public. But the words did little to ease the chill of a triple killing in the middle of a quiet town.

State Police detectives and Watertown officers are now working around the clock to piece together what led to the carnage inside the two-family home. The identities of the victims are being withheld as authorities notify relatives.

For Watertown, still scarred by its place in the Boston Marathon bombing manhunt more than a decade ago, Sunday’s bloodbath was another reminder that violence can erupt anywhere, at any time — even on a quiet street where residents thought they were safe.