The superintendent from across the street stood in the hallway in front of a locked, fourth-floor apartment door.

It was Monday afternoon on the Upper West Side and the man, identified as Lukasz Witek in an internal police report, had not heard from his relatives. He headed to the unit where his sister, Alexandra Witek, lived in the co-op building on West 86th Street near West End Avenue with her husband, Edison Lopez, who was the super of that building.

Mr. Witek, who was with Mr. Lopez’s father on the fourth floor, rapped on the apartment door several times. No answer. So they drilled a hole through the lock and peered through the opening. What they saw made Mr. Witek dial 911.

The police discovered a grim scene: Ms. Witek, 40, was lying in the hallway near the door with her two children, Lucien Lopez, 3, and Calvin Lopez, 1, police officials said. Officers saw two kitchen knives on the floor close by. All three had been fatally stabbed multiple times, and their throats were slit.

In another room, the officers discovered Mr. Lopez, 41, on a bed next to a knife, with a single, fatal slash to his neck.

Police officials said that Mr. Lopez had killed all three, then himself.

The police, relatives and tenants of the building on West 86th Street said Mr. Lopez did not show any signs of cruelty. The police never received any domestic violence reports from the apartment. Mr. Lopez did not leave a note, nor did he post anything on social media to indicate that he was a danger to his family.

He had gone with his wife and children and another family to the Intrepid Sea, Air & Space Museum in Manhattan on Monday morning, hours before they were found dead.

Mr. Lopez had been preparing to move to Hastings-On-Hudson in Westchester County, about 25 miles north of Manhattan, said Joseph E. Kenny, the assistant chief of detectives at the New York Police Department. Chief Kenny said the police are investigating whether the stress of the move contributed to the killings.

Mr. Lopez, Ms. Witek and her brother had grown up on this block where the superintendents have forged a close bond. The tight circle of people who maintain the homes of others exists within a larger ecosystem of doctors, lawyers and Broadway artists. It is also where, as in the case of the super and Mr. Lopez, sons follow the career paths established by their fathers.

On Monday evening, a crime scene van was parked in front of the co-op near the dark green awning that leads to the lobby. Some of the officers there wore hazmat suits as they wheeled stretchers past the residents, reporters and news photographers gathered outside.

Other doormen and supers had also arrived, and talked among themselves. Most said they had known the family for years, and were unsure how to process the scene.

One, Alfonso Barrera, a retired super, said he had known the Lopez family for years, and had watched Mr. Lopez grow up. Carlos Cabrera, a doorman next door, said Mr. Lopez would often take his toddler to Riverside Park, the little boy standing in the front of his electric scooter. “He had a beautiful family and he was a very nice person,” he said. “I couldn’t ever have said what’s wrong.”

Former and current tenants said Mr. Lopez was composed, considerate and attentive, often repairing their broken door hinges and dishwashers. He helped to maintain the well-kept building, and was often seen watering the plants during the summer months. Several tenants said they had heard he was preparing to take another job outside the city.

“It was the right move,” said Lynne Allen, 72, who has lived in the building since 1981, and has known Mr. Lopez since he was 10 years old.

“He was at the right age to move to a bigger building with more responsibility and more space,” she added. “They live in a one-bedroom apartment. That’s tight, you know?”

By Tuesday morning, the caution tape that had been strung outside the building was gone, as were the dozens of officers who had answered the call. A bouquet of sunflowers had been left on the stoop near two toy cars on the stoop, one red, one yellow.

From his garden apartment in an adjacent building, Charles Stroufe often saw Mr. Lopez at work and traded nods. “What everyone is saying is why,” Mr. Stroufe said a day after the killings. “Why, why, why?”

Zachary Small and Olivia Bensimon contributed reporting.