NYPD detectives are investigating the mysterious death of a 4-year-old girl who fell unconscious in her Bronx apartment with no apparent marks or injuries on her body, police said Saturday.

Cops were called to the child’s apartment near the corner of Barnes and Adee Aves. in Williamsbridge around 9:15 a.m. Friday, the day after Christmas, after family members found the girl unconscious.

EMS rushed the child to Jacobi Medical Center, where she was pronounced dead, police said.

Neighbors saw the girl’s dad burst out in tears after learning of his daughter’s death.

“The father was pacing back and forth and he broke down,” one neighbor, who only wished to be identified as Najee, told the Daily News Saturday. “I asked him if everything was all right.”

A short time later, the father knocked on Najee’s door, telling her that his daughter was dead.

“I’m just like, ‘Dang,’” she said. “I’m just sad to hear it was a young girl and during the holidays. It’s sad.”

Another neighbor called the child’s death “heartbreaking.”

The girl’s name was not immediately released by police. An autopsy has been scheduled to determine how she died.

The child’s parents declined to comment about her death.

“You want to put this on the news?” the mom told a reporter. “What’s wrong with y’all?”

Neighbors recalled that cops showed up at the child’s apartment about a week earlier, but didn’t know if it was connected to the tot’s death.

A police source with knowledge of the case said the girl did not appear injured and no wounds were found on her body.

Detectives have interviewed the family, but were awaiting the results of an autopsy before proceeding with their investigation, the source said.