A Florida state trooper has been arrested for having sex...

Police Commissioner James O’Neill promised Sunday morning to find the gunmen who turned a regularly festive Brownsville block party into a tragedy by opening fire on the crowd.

“It was not a good night in Brownsville,” O’Neill said on NY1 Sunday morning. “This is a great event, been going on a long time. I’m sure we’re going to figure out who did this and bring them to justice.”

The horrific scene unfolded just before 11 p.m. Saturday as the annual “Old Timers Day” party was wrapping up its second day with thousands in attendance.

One person was fatally shot, who police sources identified the as 38-year-old Jason Pagan, and 11 others were injured — bringing the total to 14 shooting victims overnight, including a 24-year-old who was also shot dead in Brooklyn.

But as of Sunday morning, cops were still trying to figure who pulled the trigger at the block party and why.

Chief Dermot Shea on Twitter asked for the public’s help.

Sources told The Post police believed there were two gunmen.

Dr. Burchell Marcus, 61, who told The Post he’s attended the 56-year-old block party every year, said he saw the gunfire, he helped by telling people to get to safety.

“For this individual to do something like this, it’s senseless. There’s no description for someone who does this, ” he said. “[The gunamn] was waiting outside the event to see when he could get a shot at this guy. He must have had a vendetta against him, but it was premeditated.”

Video from the aftermath of the fatal shooting shows dozens of cops comb the streets in Brooklyn hosting the decades-old block party and focus their attention on a black bag with a handgun stashed inside.

Officers with NYPD’s Crime Scene Unit photographed the bag, which was found at the foot of a tree outside a basketball court on Linden Boulevard, and pulled out a black pistol with the slide racked back, video shows.

The officers placed the gun, along with its magazine appearing still loaded with bullets, into an evidence box.

More than 100 officers were patrolling the block party when the gunfire erupted, which O’Neill said prevented more people from being shot.

“People felt emboldened enough to shoot other people,” O’Neill said. “I think after the initial shots the person or persons that did the shooting left the scene.”’

Marcus said he along with others at the party “felt safe” with the increased police presence.

“We would never have guessed with so much police presence this would happen,” he said.

Six of those injured had been released from the hospital, according to the commissioner.