Mayor Bill de Blasio on Saturday promised a “full investigation” into Friday night’s NYPD encounter in Brownsville, Brooklyn, in which an apparently mentally ill man is suspected of interfering with an arrest by throwing a metal chair at an officer’s head — he is now in a coma — and the officer shot him dead.

De Blasio also said "we need to know more about" allegations, made by the dead man's family members, that they had sought help from the city government over his troubles with mental illness.

"We obviously are awaiting a full investigation. This is a very, very serious situation. We need an investigation into everything that happened here, as per usual with the use of force," de Blasio said after casting a ballot in Park Slope, Brooklyn, at an early-voting event.

"I also want to make very clear: we’ve got an officer in a medically induced coma right now. And obviously we have a civilian who has passed away,” the mayor said.

Asked to provide the man's specific mental illness, de Blasio said: “We’re waiting to investigate that.”

The shooting of the man was the fifth in 10 days involving an officer shooting a civilian who the NYPD said endangered an officer. Four of those shootings have been deadly.

“This civilian assaulted a police officer violently and that is not acceptable. I just need to put that front and center. That is not acceptable. We do not allow assaults on our police officers,” he said of the latest one.

Det. Sophia Mason, an NYPD spokeswoman, said Saturday afternoon that the injured officer was still being treated at Brookdale Hospital Medical Center.

“He’s still in the coma,” she said.

She said the NYPD had not disclosed either the names of the injured officer or the dead man.

Friday's encounter began about 5:40 p.m., at the Goldmine Nail Salon on Mother Gaston Boulevard, when a man walked in, asked to use the bathroom, and then started urinating in the middle of the salon, according to an account later Friday by Chief of Patrol Rodney Harrison.

The proprietors flagged down two officers in a van on patrol and asked that they remove the man, whom they then questioned and learned was the subject of an outstanding warrant, Harrison said. While the officers tried to arrest the man, he resisted, and that's when another man walked in and began his own physical struggle with the officers, Harrison said.

One officer tried shocking this man with his Taser, but the stun gun was ineffective, and the man raised and threw a metal chair that struck one of the officers in the head, Harrison said, and that officer fired six times at the man. He was declared dead at the salon, he said.

The man whose public urination precipitated the events has been arrested and charged with resisting arrest, obstructing governmental administration, criminal trespass and disorderly conduct, according to Det. Michael DeBonis, an NYPD spokesman.

That man is Dewayne Hawkes, 26, of Brownsville, Mason said.

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