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An infant was dead and her baby brother was hospitalized after a witness who reported a screaming child to 911 operators said she witnessed a mother toss the boy from a second-story apartment landing in Southern California on Tuesday.

"That resident proceeded to take a look and saw a female later identified as the mother holding the child over a second story landing and proceeding to drop the child," said Upland police Capt. Marcelo A. Blanco. "She was watching that while she was on the phone."

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Police responding to the 11:30 a.m. report in the city about 35 miles from downtown Los Angeles witnessed the mother jump "face first" from the same landing, he said.

Officers who subsequently checked the apartment for other people came upon a girl believed to be about 7 months old, he said. The infant was "not breathing and unresponsive," Blanco said.

She was pronounced dead after arriving at a hospital, he said. The mother and son -- he is believed to be between 1 and 2 years old -- were also hospitalized, and both were said to be stable.

Blanco said investigators hoped the parent, identified only as a 24-year-old woman, would recover so she could be interviewed by homicide investigators. She was not under arrest. No others were found at the apartment.

Any back story is as yet unknown. Blanco said the father of the children arrived at the apartment building and expressed that he "had no clue why this would happen."

Authorities believe the girl suffered a fractured skull and internal injuries, but the exact cause of death would be determined after an autopsy, the captain said.