Surveillance video from the lobby shows Dmitriy Kanarikov leading his 3-year-old son into the 52-story apartment building at 124 W. 60th St. in Manhattan at 11:45 a.m. on Sunday.

Kanarikov was not a resident, but the video shows that he and the boy passed unchallenged by the lobby Christmas tree and moved on toward the elevators. The doorman apparently did not think to question what was so clearly a dad with his toddler son.

Some 20 minutes later, police say, a resident of a penthouse apartment saw what looked like a body fall past the window outside. A woman who lives on the 29th floor also would report seeing a plummeting body.

Shortly afterward, a caller reported to 911 that there were two bodies, one on an outcropping from the building, a smaller one on the low rooftop of an adjoining structure.

The FDNY paramedics who responded moments later declared 35-year-old Dmitriy Kanarikov dead at the scene. But little Kirill Kanarikov was still alive, and they rushed him to an ambulance, an air mask over his small face.

St. Luke’s Roosevelt Hospital is directly across the street from the back of the building, and in under a minute the boy was being rushed into the emergency room, a big hand pressing on his chest. One witness says the boy was wearing Christmas pajamas.

Back in March, a 44-year-old woman died after jumping from a building uptown while clutching her 10-month-old son. The boy had survived, but the fall was only eight stories.

The fall for Kirill was just too high. The trauma team surely hoped for a holiday miracle nonetheless, but it was not to be.

While police say they may never be entirely sure what transpired in the boy’s final minutes, the witnesses in the penthouse and on the 29th floor lead them to believe that the father took Kirill up to the roof. Kirill landed far enough away from the building that police think his father started by throwing him. The father landed closer to the building, which suggests that he had then jumped.

A booming sound heard by a man who worked at the emergency room of the hospital behind the building further suggests that the father and son plummeted from a great height. That particular noise is much too familiar to people who witnessed the jumpers from the twin towers of the World Trade Center on 9/11. The building on West 60th Street is just under half the height of the towers, but someone falling from its roof would have soon been going more than 150 miles per hour and would have taken nearly four interminable seconds to land.

In the aftermath, police determined that the boy’s parents were in the middle of a contested divorce and that the mother had been awarded custody pending a trial scheduled to commence on January 17. The father, a systems analyst originally from Ukraine and now residing in Brooklyn, was allowed to spend part of the weekend with Kirill. It was arranged that the hand-off would take place at a neutral location, in this instance the NYPD’s 17th Precinct stationhouse on the East Side of Manhattan.

As was prearranged, the father picked up Kirill there at 10 a.m. Sunday, with a requirement that he return the boy there at 1 p.m. The weather was uncommonly warm on a winter solstice in which winter had momentarily lost its bitterness, a perfect day for the father to take the boy to Central Park, maybe the zoo there. The father kept driving west.

At West 60th Street, he parked. Anybody who saw him and the boy getting out of the car might have figured they were arriving a little early for the 1 p.m. matinee of the magical Nutcracker at Lincoln Center a couple blocks uptown.

Instead, the father led Kirill into the soaring apartment building. Police believe that an acquaintance possibly lived or lives there. Police also believe that the father’s reason for choosing it was its height.

The lobby video only told police when the father and son arrived. Detectives and crime scene investigators worked into the night seeking to piece together exactly what transpired next.

“It’s one of those things you’re never going to really know,” an NYPD official later said.

In another video, one the mother posted on Facebook back in January, the father looks on as Kirill uses a red brush to clean snow off the back of a car.

“Daddy’s little helper,” wrote the mother, Svetlana Kanarikov, adding a smiley face.

She added of their son, “It took us 5 minutes to convince him that it is not necessary to sweep the entire street.”

She posted on another day, in March, “I have the best husband and son in the world.”

On the father’s Facebook page are photos he posted in September of himself and Svetlana at the beach with their son. Dmitriy had also posted a kind of declaration.

“I want to be the best dad and husband,” he wrote. “Nothing is more important to me right now.”

Whatever happened, court records indicate that divorce proceedings began by October, with Judge Jeffrey Sunshine awarding the mother “pendente lite custody,” meaning pending the outcome of the case.

The mother, who is also said to be from Ukraine, seems to have been the very best kind of mom. She changed her Facebook profile picture just last week to one of her son peering at the camera over her shoulder. She no doubt would have been expecting to collect the boy at the precinct in the afternoon and spend the coming week with him while he was off from school for the holidays.

A great mom can have no sweeter time than with a child still young enough to believe in Santa, or, in the Ukrainian tradition, Svyatyy Mykolay (St. Nicholas) or Did Moroz (Father Frost).

Dmitriy had other ideas. He seems to have entered that building with the intent of inflicting the deepest imaginable hurt on her even though that meant murdering his own son.

As the winter solstice brought the start of the longest night of the year, it also seemed the darkest along West 60th Street. The Christmas tree in the building’s lobby brought only the thought of a little boy in Christmas PJs being led past it by the very worst kind of dad.