Two uniformed NYPD officers were shot dead Saturday afternoon as they sat in their marked police car on a Brooklyn street corner — in what investigators believe was a crazed gunman’s ­assassination-style mission to avenge Eric Garner and Michael Brown.

“It’s an execution,” one law-enforcement source said of the 3 p.m. shooting.

The tragic heroes were working overtime as part of an anti-terrorism drill in Bedford-Stuyvesant when they were shot point-blank in the head by the lone gunman, identified by sources as Ismaaiyl Brinsley, 28, who had addresses in Georgia and Brooklyn.

Moments after killing the two officers, he too was dead, having turned the gun on himself on a nearby subway platform as cops closed in.

“I’m Putting Wings on Pigs Today,” a person believed to be the gunman wrote on Instagram in a message posted just three hours before the officers were shot.

“They Take 1 Of Ours…Let’s Take 2 of Theirs,” the post continued, signing off with, “This May Be My Final Post.”

The Instagram page included an image of a silver automatic handgun with a wooden handle. Another image showed the same camouflage pants and distinctive blue sneakers worn by the gunman as his body was carried from the scene on a stretcher.

He used the hashtag #ShootThePolice, along with two other hashtags referencing Garner and Brown.

Brinsley walked up to the cops’ patrol car at the corner of Myrtle and Tompkins avenues, approaching from the sidewalk.

Witnesses told police that Brins­ley wordlessly blasted into the patrol car’s front passenger-side window.

Then he stood stock still for a few moments, fleeing into the subway only when he heard the sirens of a second police car.

Once in the subway, “they engaged the guy and he did himself,” one investigator said.

Brinsley was a fugitive who had just murdered his girlfriend in Baltimore Saturday morning, sources told The Post....