The gunman who ambushed and killed two New York City police officers in their patrol car Saturday afternoon in Brooklyn, posted messages on social media that vowed retaliation for the deaths of Michael Brown and Eric Garner, authorities said.

Ismaaiyl Brinsley, 28, wrote on his Instagram account: "I'm putting wings on pigs today. They take 1 of ours, let's take 2 of theirs," hours before the killings, Fox News reported.

According to police reports, Brinsley approached the police car at approximately 2:45 p.m. local time and opened fire, striking officers Rafael Ramos and Wenjian Liu in the head.

New York City Police Commissioner William Bratton said the two officers were deliberately targeted. "They were, quite simply, assassinated, targeted for their uniform and the responsibility they embraced," Bratton said. "Both were ambushed and murdered."

After shooting Ramos and Liu in the Bedford-Stuyvesant neighborhood of Brooklyn, Brinsley was chased by officers into a nearby subway station, where the gunman shot himself in the head on a train platform, NPR reported.