The thug who fatally shot a young woman execution-style at the J’Ouvert festival in Brooklyn was arrested Tuesday — while drunkenly driving on three wheels and with a Caribbean flag wrapped around his bloody hand, police said.

The 20-year-old suspect, Reginald Moise, was picked up at 8 a.m. for driving drunk, and police then linked him to the shooting of 22-year-old Tiarah Poyau at the pre-West Indian Day Parade event, police said.

He told pals before his arrest, “I think I shot somebody on the parade route, and I didn’t know the gun was loaded,” said NYPD Chief of Detectives Robert Boyce.

“We then begin to speak to him in the 71st detective squad, where he goes on to say he thinks he shot somebody. ‘The gun went off. I thought it wasn’t loaded. I’m not sure.’ And then he requested an attorney.”

The gun used in the killing, a Glock 9mm, was recovered at his girlfriend’s apartment, police said. Ballistics testing matched it to shells found at the scene, Boyce said.

“On the scene, we find an aluminum-jacket 9mm. It’s not particularly common but we’ve seen them before,” Boyce said. “This aluminum jacket 9-millimeter is also at the crime scene on Empire Boulevard. We start to get suspicious. We then send it down to the lab, and they make the comparison. It’s the same gun.”

Just before her death, Poyau had been walking with three friends. Her pals told police they heard a shot and then saw her fall, police said.

Poyau, a St. John’s University student, was struck in the eye “at close range” around 4:15 a.m. at Empire Boulevard and Franklin Avenue, according to law enforcement sources.

“This young lady is just a stellar person. … No issues in her life whatsoever,” Boyce said.

After the shooting, the suspect went to his girlfriend’s home, where he was visibly drunk, police said.

“At this point, he fires two rounds into the wall, and then he breaks a window or a mirror, and he cuts his hand. There’s blood on the mirror,” Boyce said.

At least one of the bullets went into the house next door, and the neighbor called the cops, and the man fled in a car, sources said.

When Moise was stopped for drunken driving while missing a car wheel, “his hand is cut up, and it’s tied off with a Caribbean flag,” Boyce said.

“It all starts to fit together.”

Moise was charged with second-degree murder and depraved indifference, authorities said.

Less than an hour before Poyau was killed, another reveler, Tyreke Borel, 17, was shot in the chest and died at Kings County Hospital.