The 14-year-old mugger suspected of killing Barnard College freshman Tessa Majors likely snapped and stabbed her to death because she bit his finger, law enforcement sources told The Post on Friday.

Majors, 18, was fighting for her life when she was set upon by the middle-school criminal and his two young buddies — and in a desperate bid to survive, she clamped down hard on the finger of the knife-wielding young mugger, who then flew into a rage, stabbing her multiple times, sources said.

“She bit him, and that is why he stabbed her,” a source said, citing the current theory detectives are working on.

One of the 14-year-old suspect’s alleged accomplices told cops right after the murder that Majors bit the teen’s finger, court papers show.

And in doing so, Majors also may have provided cops with the forensics to solve her murder. Investigators believe that her bite on the boy’s finger drew blood — and that blood may have been left behind on Major’s body.

The suspected stabber submitted Thursday to a Manhattan court-ordered DNA test, police officials said. Early next week, tests will reveal the boy’s DNA profile, authorities said, which will be compared with samples taken from swabs of Majors’ mouth.

Sources say the teen’s DNA will also be compared against any stray cells that rubbed off on Majors’ clothing during the struggle, which happened in Morningside Park on the evening of Dec. 11.

Meanwhile, the suspected stabber remains free without charges, pending the results of the DNA testing.

He was on the lam for more than two weeks before cops nabbed him in the Bronx on Thursday. He was brought in for questioning but didn’t say anything on the advice of his lawyers. Investigators believe he may have stayed underground until his finger healed, according to a source.

“He must not have had a bite mark” on his finger when he was taken in, a source familiar with the case said. “If he did, that would have been enough corroborating evidence to hold him.”

“He’s funny — he could tell a good joke to lighten up the mood,” a 12-year-old neighbor of the 14-year-old said Friday.

“I know he hangs around with not the right people at times,” she added. Still, “Nobody would ever think he would do anything like this,” she said.

“We don’t think he did it.”

So far, only Zyairr Davis, 13, is in custody and charged with the Dec. 11 murder, in which police believe three young muggers took part.