A policeman puts up crime scene tape at the scene of a shooting where two New York Police officers were shot dead in the Brooklyn borough of New York, December 20, 2014. [Photo/Agencies]

A gunman ambushed and fatally shot two New York City police officers on Saturday and then killed himself, police said, and a social media post indicated it may have been in revenge for the police chokehold death of an unarmed black man.

The two police officers were identified as Wenjian Liu, 32, and Rafael Ramos, 40, according to William Bratton, New York police commissioner at the press conference on Saturday. Liu had been married for two months. Ramos had a 13-year-old son.

The officers were killed without warning and at close range as they sat in their squad car in the Bedford-Stuyvesant section of Brooklyn, said Bratton at the news conference flanked by New York City Mayor Bill de Blasio.

"Although we're still learning the details, it's clear that this was an assassination, that these officers were shot execution style," said de Blasio.

New York police have come under intense pressure in recent weeks, with protests erupting after a grand jury declined this month to charge a white police officer involved in the chokehold death of Eric Garner during an arrest attempt in July in the borough of Staten Island.

Demonstrations over Garner's death came on top of protests around the country over another grand jury's decision in November not to indict a white police officer who fatally shot an unarmed black teenager in Ferguson, Missouri. President Barack Obama, who was briefed on the New York killings while on vacation in Hawaii, has had to calm eruptions of anger over police brutality against minorities.

The killings were the first time New York City police officers have been killed by gunfire since 2011 and were bound to fuel anger among some police against de Blasio, who has had a prickly relationship with law enforcement.

Bratton identified the gunman as Ismaaiyl Brinsley, 28, and said he took a shooter's stance on the passenger side of the squad car, opening fire with a silver semi-automatic handgun. He then fled into a nearby subway station and died there from a self-inflicted gunshot wound to the head, Bratton said.

INSTAGRAM POSTING

An online posting suggested a possible link between Brinsley, who was black, and anger over the death of Garner.

Screenshots taken by various media showed an Instagram account attributed to Brinsley with a picture of a black man with wire-rimmed glasses and a separate picture of a silver pistol.

The account, using the slang insult pig for police, said: "I'm Putting Wings On Pigs Today. They Take 1 Of Ours ... Let's Take 2 of Theirs."

The post included hashtags for Eric Garner and for Michael Brown, the teenager who was shot dead in August in Ferguson.

Instagram said the account attributed to Brinsley had been deleted.

Bratton was asked whether there was a link between Brinsley and the weeks of protests over law enforcement, and said this was under investigation. He added:

"There has been ... a very strong anti-police, anti-criminal justice system, anti-societal set of initiatives under way and one of the unfortunate aspects sometimes is some people get caught up in these and go in directions they should not."

He said police would investigate whether Brinsley had been part of protests in New York and in Atlanta, his last place of residence, over the Brown and Garner killings.

Brinsley had shot and seriously wounded his ex-girlfriend in Baltimore County, Maryland, early on Saturday before traveling to Brooklyn, where he had connections, the police commissioner said.

Attorney General Eric Holder called the killings of the police officers "barbarism."