An infant girl died Tuesday in Upland after being found unresponsive at an apartment complex, where police said her mother threw her toddler brother from a second-story balcony and then jumped herself.

The girl was found “not breathing and unresponsive” when officers entered the apartment, Upland police Capt. Marcelo Blanco said at the scene Tuesday afternoon. She suffered a skull fracture and other possible injuries.

Police said Tierra Rose Ortega, 24, jumped off the landing. She was arrested after being released from a hospital on Tuesday evening.

Ortega was booked into the West Valley Detention Center on suspicion of assault on a child younger than eight, attempted murder and murder, according to court records. Court records state Ortega is ineligible for bail

Her police booking photo showed injuries to her face.

Detectives were at the scene into the night working to establish a motive.

Both children and their mother were transported to the hospital, where the girl died. The boy remained hospitalized and was reported to be stable. The mother was also stable and was speaking to investigators in the hospital.

Neighbors say they called police about 11:30 a.m. to the complex in the 600 block of N. Wedgewood Avenue, when they saw the woman throw the boy from a second-floor landing. When officers arrived, the mother jumped to the concrete below.

Neighbor Kaleigh Calderon said she doesn’t know the family, although she did say she and the woman were pregnant at the same time.

“That’s terrible. As a mother, that’s terrible,” Calderon said. “You never know what is happening.” She said the family was quiet.

The father, who was not home at the time of the incident, was also questioned by police, authorities said.

Some neighbors who watched Tuesday afternoon as police worked inside the complex, said they were shocked at what took place. The apartment complex sits in a quiet cul de sac off 11th Street in Upland.

“Walking up on them doing CPR on babies, it’s shocking,” said one man, who called himself a friend but declined to be identified.

Staff writer Robert Gundran contributed to this report.