NEW YORK - The gunman who fatally ambushed two New York City police officers in their squad car had a long criminal record, a hatred for police and the government and an apparent history of mental instability that included an attempt to hang himself a year ago, police said Sunday.

Ismaaiyl Brinsley approached people on the street moments before opening fire and asked them to follow him on Instagram, then told them to "watch what I'm going to do," Chief of Detectives Robert Boyce said.

A portrait of the shooter emerged as big-city police departments and union leaders warned officers to change up their routines and insist on extra backup a day after Brinsley carried out what he portrayed online as retaliation for the slayings of black men at the hands of white police.

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 in his online threat, or simply latched on to the cause for the final act of a violent spree. Police said he had no gang affiliation.

Brinsley, 28, had at least 19 arrests in Georgia and Ohio and a troubled childhood so violent his mother was afraid of him, police said. Brinsley had also ranted online about police and government and expressed despair about his own life, Boyce said.

Boyce said Brinsley's mother believed he had undiagnosed mental problems and may have been on medication later in life but detective said they were still trying to determine if he had a mental illness.

Brinsley approached a squad car from behind and fired four shots, fatally wounding officers Rafael Ramos and Wenjian Liu, before running into a subway station and ultimately killing himself.

Hours earlier, Brinsley had shot and wounded his ex-girlfriend at her home outside Baltimore then made threatening posts online, including a vow to put "wings on pigs," and references to a pair of police-involved deaths that have sparked racially charged protests across the country.

The killings dramatically escalated tensions that have simmered for months over the deaths of young black men.

The siege mentality was evident in several memos circulating among the rank and file at the 35,000-officer New York Police Department, the nation's largest.

A union-generated message warned police officers they should respond to every radio call with two cars - "no matter what the opinion of the patrol supervisor" - and to not make arrests "unless absolutely necessary." The president of the detectives' union told members in a letter to work in threes when out on the street, wear bulletproof vests and keep aware of their surroundings.

"Cowards such as yesterday's killer strike when you are distracted and vulnerable," the letter read.

Another directive warned officer in Newark, New Jersey not to patrol alone and avoid people looking for confrontations with them. A the same time, a memo from an NYPD chief asked officers to avoid fanning rage within the ranks by limiting comments "via all venues, including social media, to expressions of sorrow and condolence. ... Even in our most difficult times, we will remain consummate professionals."

Brinsley traveled by bus to New York City on after wounding his girlfriend. Before the shootings, Brinsley wrote on an Instagram account: "I'm putting wings on pigs today. They take 1 of ours, let's take 2 of theirs," officials said. He used the hashtags Shootthepolice RIPErivGardner (sic) RIPMikeBrown - references to the police-involved deaths of Garner and Brown.

The slayings come at a tense time; Police in New York and nationwide are being criticized for their tactics, following the July death of Garner, who was stopped on suspicion of selling loose, untaxed cigarettes. Amateur video captured an officer wrapping his arm around Garner's neck and wrestling him to the ground. Garner was heard gasping, "I can't breathe" before he lost consciousness and later died.

Demonstrators around the country have staged die-ins and other protests since a grand jury decided Dec. 3 not to indict the officer in Garner's death, a decision that closely followed a Missouri grand jury's refusal to indict a white officer in the fatal shooting of Brown, an unarmed black 18-year-old.

Court records in Georgia show that Brinsley had several run-ins with the law there in recent years. Charges included gun possession, shop-lifting and theft.

Bratton and De-Blasio attended Mass at St. Patrick's Cathedral, where Cardinal Timothy Dolan called for calm. He asked the police commissioner to tell his troops that "we love them very much, we mourn with them, we need them, we respect them, we're proud of them and we thank them."

Brooklyn Borough President Eric Adams visited a makeshift memorial at the site of the slayings, calling on protest organizers to "hold off on any type of protest until these officers are laid to rest in a peaceful manner."

At an appearance with the Rev. Al Sharpton where he denounced violence against police, Garner's mother expressed her dismay.

"I'm standing here in sorrow about losing those two police officers that was definitely not our agenda," Gwen Garner said.

"We are going in peace and anyone who's standing with us we want you to not use Eric Garner's name for violence because we are not about that," she said. "These two police officers lost their lives senselessly and our condolence to the family and we stand with the families."