The man who shot and killed two New York City police officers as they sat in their patrol car had posted angry messages on social media, ranting against the government and police, New York Police Department's Chief of Detectives Robert Boyce said Sunday.

Ismaaiyl Brinsley, 28, also posted messages of self-loathing and despair and made reference online to Michael Brown and Eric Garner, African-American men who were killed by police.

"I'm Putting Wings On Pigs Today. They Take 1 Of Ours, Let's Take 2 of Theirs," an Instagram post read, authorities said.

Brinsley had a lengthy criminal record. He was arrested 15 times in Georgia for assorted crimes and arrested four times in Ohio, Boyce told reporters Sunday.

He was in prison in Georgia between August 2011 and July 2013 for criminal possession of a weapon, Boyce said.

Brinsley had an address connected to him in Georgia but that appears to be his sister's home, and they are estranged, Boyce said. The gunman's connection to Brooklyn is that his mother lives there, but they are also estranged, the chief of detectives said.

On Saturday afternoon, Brinsley fired four rounds at officers Rafael Ramos and Wenjian Liu in the Bedford-Stuyvesant neighborhood. The officers, who were hit in head, were assigned from their normal downtown Brooklyn beat to an area of the borough with a high crime rate, authorities said.

New York City Police Commissioner William Bratton said the officers were shot "with no warning, no provocation."

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