NEW YORK — The killer who strode up to a mobile police command post and put a bullet in an officer’s head Wednesday had ranted in a Facebook video last September about law officers killing and abusing people and warned them to leave him alone or “we gonna do something.”

“I’m not playing, Mr. Officer. I don’t care about 100 police watching this,” 34-year-old ex-convict Alexander Bonds said, adding: “It’s time for people to wisen up.”

Ten months later, Bonds went up to the RV-like command post in the Bronx and ambushed Officer Miosotis Familia, shooting her through the passenger-side window as she wrote in her notebook around 12:30 a.m.

New York City Police Commissioner James O’Neill said Familia was “assassinated in an unprovoked attack on cops.”

Familia’s partner frantically radioed for help, and officers caught up with Bonds about a block away and killed him in a hail of about 20 bullets when he pulled a stolen revolver, police said. He didn’t get off a shot, authorities said. The burst of gunfire as the Fourth of July wound down was initially mistaken by some for fireworks.

Familia, 48, was a 12-year veteran of the force who spent her entire career with the department in the high-crime Bronx precinct. The command post there had been set up and staffed around the clock since a triple shooting in March.

“She was on duty serving this city, protecting people, doing what she believed in and doing the job she loved,” Mayor Bill de Blasio said.

Police said they were trying to establish the motive for the shooting.

While tensions have been running high in recent years between police and black people around the country, there was no immediate indication the killing had a racial dimension. Bonds was black; Familia was black and Hispanic, her family having come from the Dominican Republic. She apparently had no previous contact with him.

Still, the attack recalled the 2014 ambush killings of two New York City officers who were gunned down in their cruiser by a man who had announced online moments before that he was planning to shoot two “pigs” in retaliation for the police chokehold death of Eric Garner in New York. The gunman, 28-year-old Ismaaiyl Brinsley, then killed himself.

Attorney General Jeff Sessions called Familia’s killing a “murder in cold blood.”

“Sadly, it is the latest in a troubling series of attacks on police officers over the past two years. These attacks must stop,” he said in a statement.