Police departments on Long Island and across the country are on high alert after two NYPD officers were shot and killed in Brooklyn Saturday by a gunman who has been described as "anti-cop" by investigators.

Officials say the 28-year-old gunman, Ismaaiyl Brinsley, ambushed the two NYPD officers as they sat in their patrol car in Brooklyn's Bedford-Stuyvesant neighborhood. Brinsley fatally shot both of them before he fled into a nearby subway station and killed himself on the platform.

The officers have been identified as Officer Rafael Ramos, 40, and Officer Wenjian Liu, 32, both of Brooklyn. Ramos, a three-year veteran of the force, was married and had a 13-year-old son and another in college. Officer Liu, a seven-year veteran, married his wife about two months ago.

In the wake of the shooting, police unions are warning their officers to wear their bulletproof vests and to respond to all calls with at least two cars. They are also advising officers not to make arrests unless absolutely necessary, and to avoid putting inflammatory posts on social media.

Before the shootings in Brooklyn, investigators say Brinsley posted comments that were "very anti-police" on social media accounts, including one comment where he allegedly wrote, "I'm putting wings on pigs today," and said "They take 1 of ours, let's take 2 of theirs." Officials say he also used the hashtags #Shootthepolice, #RIPErivGardner [sic] and #RIPMikeBrown, references to two black men who died during run-ins with police.

Detectives say moments before the attack, Brinsley walked up to two people on the street and asked them for their gang affiliation, to follow him on Instagram and to "watch what I am going to do."

Investigators say Brinsley, who had a lengthy criminal record, was a fugitive wanted for shooting his ex-girlfriend in Baltimore early Saturday morning before coming to New York and killing the officers. The 29-year-old ex-girlfriend is expected to survive.

The slayings came at a time when the city is dealing with heightened tensions between law enforcement, the public and some city officials. Mayor Bill de Blasio has drawn strong criticism from the head of the city's police union, and some have accused the mayor of caring more about the rights of demonstrators than supporting officers.