New York City police officer Darren Cox, right, accompanied by fellow officers, leaves flowers at a memorial in the Brooklyn borough of New York City on Dec. 21, 2014.
Mark Lennihan—AP
December 21, 2014 5:02 PM EST

Moments before he ambushed and killed two New York police officers, gunman Ismaaiyl Brinsley told passersby to follow him on Instagram and then said, “Watch what I’m going to do,” investigators said Sunday.

The disclosure came as police sketched a timeline of Brinsley’s activities in the hours before he shot and killed Officers Wenjian Liu and Rafael Ramos in Brooklyn on Saturday. Brinsley later killed himself. Investigators said Brinsley, 28, shot his ex-girlfriend at her Baltimore apartment after an argument, then took a bus to New York.

On social media accounts, he occasionally ranted against police and government, investigators said.

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