The father of a 21-year-old man who was shot and killed by police as he swung a knife at them on Roosevelt Island over the weekend said his family was “not OK.”

“Please we are not able to talk with anybody,” the grieving dad told The Post on Saturday, one day after his son was shot by police.

“We are not OK,” he continued when asked how the family was holding up.

Police went to the building at 540 Main Street around 11:30 p.m. Friday on multiple 911 calls from the man’s relatives, who said he was threatening one of them with a knife — and that he was off his medication and under the influence of marijuana, cops said.

Three officers got into one of the building’s two elevators and a fourth propped the front door open to allow more police to respond. 

As the officer was doing that, the knife-wielding man came out of a different elevator and charged at the officer.

That cop ran outside.

The man, who hasn’t been publicly identified, then turned his attention to the elevator the three police officers were monitoring, and charged at them with the large knife, cops said.

The officers tried to shock him with a Taser before shooting him, police said.

It was clear how many of them fired or how many times, cops said.

The officers tried to help the man but he couldn’t be saved.

The entire incident unfolded in minutes, officials said.

Kevin Kelly, lives on the first floor, and heard the gunshots.

“I heard the shots, bang, bang, bang, like six or seven,” he said.

“I thought it was fireworks. Then I heard the ambulance coming five minutes later.”

He said soon after that he received a text saying Main Street was closed.

“Then, I went to walk my dog, but they wouldn’t let me out,” he said.

“I had to go around to the back. I saw the tape. It must have been at least 10 cop cars and ambulances.”

He said there are a lot of mentally ill people in the building who need help.

Brittany Johnson was coming home around 4 a.m. and saw crime scene tape.

“Basically they taped the whole building 540, the front of the whole building was all taped up,” she said.

“I saw the police tape and I asked the police officer, ‘I live here, could I come in the building?’” she recalled.

“He was like ‘No, just go around to the other building.”