Big-city police departments and union leaders around the country are warning the rank and file to wear bulletproof vests and avoid making inflammatory posts on social media in the days after a man ambushed two officers and shot them to death inside their patrol car.

The slayings of Officers Rafael Ramos and Wenjian Liu on Saturday afternoon in Brooklyn heightened fears about the safety of law enforcement officials nationwide, though authorities haven’t said if they believe 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.

The slayings come at a tense time as police nationwide are being criticized following Garner’s death in a New York officer’s chokehold and Brown’s fatal shooting in Ferguson, Missouri. Protests erupted in recent weeks after grand juries declined to charge the officers involved.

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 warning officers about the New York City slayings and added that the department had issued several alerts following the Ferguson grand jury’s decision.

At a news conference in New York on Sunday, Chief of Detectives Robert Boyce detailed Brinsley’s long criminal record, hatred for police and the government and apparent history of mental instability that included an attempt to hang himself a year ago.

Brinsley had at least 19 arrests in Georgia and Ohio, spent two years in prison for gun possession and had a troubled childhood so violent that his mother was afraid of him, police said. He ranted online about authority figures and expressed “self-despair and anger at himself and where his life was,” Boyce said.

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

“He shot me! I don’t want to die. I don’t want to die,” Thompson screamed, according to neighbor Yevette Seay. She told The Associated Press she called 911 as Thompson lay on the floor, clutching her stomach. Authorities have said Thompson is expected to survive.

After leaving Baltimore, authorities said, Brinsley took a bus north to New York City and used Thompson’s phone to write on Instagram: “They take 1 of ours, let’s take 2 of theirs.” He ended the post with references to the Brown and Garner cases.

Once in New York and shortly before he opened fire on the officers, Brinsley walked up to people on the street and asked them to follow him on Instagram, then told them, “Watch what I’m going to do,” Boyce said. Then Brinsley approached the squad car and fired four shots, killing the policemen. He ran into the subway station and committed suicide.

At the site of the shooting, a makeshift memorial with an NYPD baseball cap, flowers, teddy bears and candles lay Sunday in remembrance of the officers.

Police Commissioner William Bratton placed flowers at the memorial and visited both officers’ families after attending Sunday Mass at St. Patrick’s Cathedral with Mayor Bill de Blasio.

De Blasio’s relationship with rank-and-file police has been so fractious that their union president recently suggested officers sign a petition telling the mayor not to attend their funerals if they died on duty. But one of Ramos’ relatives said the family would welcome a visit from the mayor, who visited both officers’ families at the hospital Saturday but did not see them Sunday.

“If he wants to come and show support, by all means, we’ll accept that,” said Ramos’ cousin Ronnie Gonzalez.

Ramos’ 13-year-old son, Jaden, said in a Facebook post that Saturday was the worst day of his life.

“Today I had to say bye to my father,” Jaden wrote. “He was the best father I could ask for. It’s horrible that someone gets shot dead just for being a police officer. Everyone says they hate cops but they are the people that they call for help.”

(AP)