Police in New York City and elsewhere are on guard after the deadly ambush of two officers sitting in their squad car on a Brooklyn street.The gunman who opened fire on Saturday afternoon had posted online that he planned to kill police in retaliation for the police-involved deaths of Eric Garner in NY and Michael Brown in Ferguson. He fatally shot himself after killing the officers. Investigators sought to determine whether Ismaaiyl Brinsley really had a deep sympathy for the police killings protests or simply latched on to the cause for the final act of a violent. Police say Brinsley ambushed the officers after shooting his girlfriend.