An Upper West Side man brutally killed his wife and two sons — one an infant — before taking his own life Monday, according to police sources.

Cops found the bodies of the 41-year-old suspect, his 40-year-old wife, and their sons, ages 1 and 3, in an apartment on W. 86th St. near Riverside Drive around 2:50 p.m.

The adult victims had their throats slit and the children suffered stab wounds to their little bodies, according to police.

Paramedics pronounced all of the victims dead at the scene, cops said.

Their names were not immediately released. But a neighbor identified the killer as the super of the building who lived with his family in the fourth-floor apartment where the grisly scene was found.

The resident, Jeff Kimmel, said the super’s father, himself a former superintendent at the same building, had come to visit his grandchildren earlier Monday.

After being unable to get into the apartment, he forced the door open, according to cop sources. Emergency services were called when he saw blood on the floor, they added.

Firefighters assisted officers in gaining entry to the unit, where the man’s body was found in bed and the woman’s in a hallway, the sources said.

The children were found dead in the living room. At first, cops identified the children as a boy and a girl, but it was later determined that the victims were both boys, cops said.

The killer used multiple knives during the carnage, according to police.

The dead man’s father and mother were seen visibly shaken in the building lobby.

“They probably were gonna either baby-sit or hang out or do something,” Kimmel said of the parents of the alleged killer. “Everybody’s really shocked.”

Kimmel said he’d known the deceased man for more than two decades, describing him and his family as well-liked by residents.

“Really nice guy. Everybody likes him. He’s solid,” the neighbor said. “He has a lovely wife from what I gather. Two lovely young kids. Great family. I couldn’t imagine what this could be.”

Residents enjoyed watching their super’s family grow over the past several years, according to Kimmel, who said the super had boasted of getting a new job in Westchester that would allow him to better provide for his wife and two kids.

“We were happy for him when he got married and had his kids,” said Kimmel. “He seemed happy. He said, ‘Yeah, it’s a good move for the family.’”

The now-dead super had lived in the W. 86th St. co-op since he was a boy, and many longtime residents watched him grow up before he took over the reins from his father.

“I knew [the super] since he was 10 years old when he moved into the building,” said Lynne Allen, a decades-long resident. “He was my cat-sitter when he was a young teen and just a great guy. Great, lovely family and adorable little children.”

Allen described the victims as an outwardly perfect family with a beautiful young mother, two happy children and a father excited to begin working at his new job.

“She was a gorgeous young woman,” she said. “They seem very happily married and then they had this lovely family and now they’re moving to Westchester and everything was perfect.”

Residents at the building were shocked to learn of the heinous killings, with Kimmel saying he assumed that the attack came from outside the building — not from within the family, as police suspect.

“The first thing we’re thinking is, ‘Who could’ve come into our building and done this?’” said Kimmel. “I think it’s crazy.”

The W. 86th St. slayings occurred about a half-mile away from where the body of 74-year-old Maria Hernandez was discovered bound and gagged in her W. 83rd St. apartment near Columbus Ave. on Jan 18.

Police cuffed Lashawn Mackey, a former superintendent of the building, and homeless man Terrence Moore for Hernandez’s murder. Both men face burglary and murder charges.