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Ismaaiyl Brinsley, who ambushed and "executed" two NYPD officers while they were sitting on their squad car on Saturday, reportedly approached bystanders on the street minutes before opening fire and asked them to "watch what I'm going to do," according to Chief of Detectives Robert Boyce.

Boyce told the Associated Press that Brinsley also asked bystanders to follow him on Instagram, where he'd previously posted photos of the alleged murder weapon.

A 2009 handout photo of Ismaaiyl Brinsley
A 2009 handout photo of Ismaaiyl Brinsley

A history of problems: New York City investigators are slowly building a profile of Brinsley, who reportedly ranted about murdering police officers before shooting his girlfriend in Baltimore, executing the two police officers in New York and committing suicide inside a subway station. 

According to the Associated Press, "the gunman also had a history of mental instability, which included an attempt to hang himself a year ago. Brinsley's mother said she believed he may have been on medication at some point, but it remains unclear if he had mental illness. Investigators are still trying to ascertain a motivation for Brinsley's attack. Authorities said Brinsley also spoke out against police and government online, expressing "self-despair and anger at himself and where his life was."

Brinsley had mentioned Michael Brown and Eric Garner, the two unarmed black men killed in engagements with police whose deaths sparked national outcry, in now-suspended Instagram posts. Investigators were trying to determine if Brinsley had taken part in any protests over the deaths of Michael Brown and Eric Garner, whose names he invoked online, "or simply latched on to the cause for the final act in a violent rampage," the Associated Press reports.

Police unions in New York have accused Mayor Bill de Blasio of stirring up anti-police sentiment following his candid remarks about his son Dante in the wake of the Eric Garner grand jury decision. Former New York Mayor Rudy Giuliani went so far Sunday as to accuse President Barack Obama of "months" of anti-police propaganda, implying that the president is responsible for Brinsley's savagery.

Editors Note: Mar. 2, 2015 

An earlier version of this article cited Associated Press reporting, but did not include quotations around the cited passage. The story has been updated to fully attribute the Associated Press' language.