Slain Barnard College student Tessa Majors’ accused teen attacker said an accomplice slashed the Barnard College student so viciously that the young teen saw “feathers come out of her jacket,” a Manhattan detective testified Friday.

Zyairr Davis, 13, told cops that his pal “dropped a knife” as the pair and a third male suspect pounced on the 18-year-old college freshman in Morningside Park on Wednesday evening during a robbery-gone-awry, said NYPD Detective Vincent Signoretti, who helped interview Davis.

“Mr. Davis picked [the knife] up and handed it back to [his friend], and then Mr. Davis watched his friends grab the victim, put her in a chokehold and remove items from her pocket,” the cop said.

Then Davis “saw the victim get stabbed. He saw feathers come out of her jacket, and them all three of them ran out of they park … and they went home,” Signoretti said.

Majors futilely tried to fight off her attackers, the detective said.

Davis said his friend told him “that Tessa Majors bit him on the finger,” Signoretti said.

Surveillance footage shows the trio entering the park, but there is not footage of the slaying, the officer said.

Law enforcement sources have said Davis’ accomplices are two boys he goes to middle school with, including a 14-year-old also now in custody.

Davis, who lives a few blocks from the Morningside Park crime scene, bit his bottom lip and looked on nervously before his appearance in Manhattan Family Court, where he appeared in a navy-blue sweatshirt, black sweatpants and black and white Air Jordan sneakers.

He was charged with murder and ordered held without bail.

Asked by a court officer if he would swear to tell the truth during the hearing, the young teen replied, “I swear.”

The officer asked the teen to repeat it, saying he couldn’t hear him.

“I swear,” the boy repeated more loudly.

“My name is Zyairr Davis, and I’m 13,” the kid added.

Additional reporting by Joe Marino and Tina Moore