Wear bulletproof vests. Avoid posting inflammatory statements on social media. Don't make arrests unless you have to. Don't patrol alone.

Those are some of the warnings police departments and unions around the country are giving to the rank-and-file after two NYPD officers were ambushed and shot in their patrol car in broad daylight Saturday afternoon. 

The murders of Officers Rafael Ramos and Wenjian Liu in Brooklyn heightened fears about the safety of law enforcement officials nationwide, though there is no evidence any threats are imminent. The gunman, 28-year-old Ismaaiyl Brinsley, had vowed in an Instagram post to put "wings on pigs" as retaliation for the slayings of black men at the hands of white police.

Brinsley was black; the slain New York Police Department officers were Hispanic and Asian.

Investigators are 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 onto the cause for the final act in a violent rampage. Protests erupted in recent weeks after grand juries declined to charge officers involved in Garner's death following a New York officer's apparent chokehold and Brown's fatal shooting in Ferguson, Mo.

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After the officers' killings, a union-generated message at the 35,000-officer NYPD warned officers that they should respond to every radio call with two cars — "no matter what the opinion of the patrol supervisor" — and 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.

Another directive warned officers in Newark, New Jersey, not to patrol alone and to avoid people looking for confrontations. At the same time, a memo from an NYPD chief asked officers to limit their comments "via all venues, including social media, to expressions of sorrow and condolence."

In Philadelphia, Police Commissioner Charles Ramsey urged the leaders of protests over the deaths of Garner and Brown to "call for calm and not let this escalate any further." In Boston, Police Commissioner William Evans said police issued an alert to officers to warn them about the New York City slayings and added that the department had issued several alerts following the decision by the Ferguson grand jury.

At a news conference Sunday afternoon, New York City Chief of Detectives Robert Boyce told reporters that Brimsley had numerous run-ins with law enforcement.had been arrested 15 times in Georgia, and four in Ohio for assorted crimes. He ranted online about authority figures and expressed "self-despair and anger at himself and where his life was," Boyce said. 

Right before the shooting, Brinsley talked to two people on the street, asking them for their gang affiliation, urging them to follow him on Instagram, and then declaring “watch what I’m going to do,” Boyce added.

The silver handgun used in Saturday’s shooting in the Bedford-Stuyvesant neighborhood was bought at a pawn shop in Georgia in 1996 and authorities are now working with the ATF to determine how it ended up in Brinsley’s hands.

They said he traveled frequently between the South and New York, where he fathered a child in Brooklyn, and had been in the city earlier in the week. Brinsley was born in Brooklyn.

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. He attempted to hang himself a year ago, police said. His mother had told authorities that she was scared of her son and he had a troubled childhood, often acting violent.

Hours before shooting the officers, Brinsley had shot and wounded his ex-girlfriend, Shaneka Thompson, at her home in Baltimore.

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Thompson's neighbor Yevette Seay told The Associated Press that Thompson's screams for help early Saturday jolted her out of bed.

"He shot me! I don't want to die. I don't want to die," Seay recalled Thompson saying.

Seay called 911 as Thompson lay on the floor, clutching her stomach. Authorities have said Thompson is expected to survive.

Brinsley then traveled north on I-95 to New York City on a Bolt bus and arrived in Manhattan, according to Boyce.

Authorities are looking for information as to what Brinsley was up to shortly after noontime and before the shooting.

“I'm putting wings on pigs today. They take 1 of ours, let's take 2 of theirs," Brinsley wrote on his Instagram account Saturday hours before the killings, two city officials with direct knowledge of the case confirmed for The Associated Press. He used the hashtags Shootthepolice RIPErivGardner (sic) RIPMikeBrown. The post also included an image of a silver handgun and the message, "This may be my final post.” The post had more than 200 likes but also had many others admonishing his statements.

Shortly after the shooting, Brinsley fled to a nearby subway station, where he shot himself in the head as a subway train door full of people closed.

Two unnamed Con Edison workers tried to stop Brinsley before he entered the station, but retreated.

When the workers confronted Brinsley on the street, he pointed his gun at him and asked them, “You want some of this?” before they backed off, the New York Post reports.

The two workers alerted police to Brinsley’s location.

“That’s how the cops knew he was in there,” one law enforcement source told the New York Post.

Boyce said Sunday that the police "can't thank those two individuals enough.”

Ramos' cousin, Ronnie Gonzalez, said the family has already forgiven the gunman.

"He's in the hands of God now," Gonzalez said. "We don't believe in vengeance, we just forgive."

Lucy Ramos, his aunt, asked for a “peaceful co-existence” and hopes that the community can move forward from the shooting.

A candlelight vigil was held at the scene of the shooting on Sunday night.

Ramos was married with a 13-year-old son and had another in college, police and a friend told the Associated Press. He had been on the job since 2012 and was a school safety officer.

The New York Yankees will pay for the education of both children, the New York Daily News reports

Liu had been on the job for seven years and got married two months ago.

The Associated Press contributed to this report.