The gunman who fatally shot two New York City police officers watched a protest over police-involved deaths earlier this month, but did not participate.

Investigators say Ismaaiyl Brinsley was at a protest in New York's Union Square on Dec. 1, before a grand jury decided against charging a white officer in the chokehold death of Eric Garner, an unarmed black man.

Police said Brinsley recorded part of the protest on his phone, like other bystanders.

In online posts before the shooting, police say Brinsley vowed to kill police officers in retribution for the deaths of black men at the hands of white officers.

Investigators are trying to determine if Brinsley latched onto the cause for the final act in a violent rampage.