Authorities identified the slain infant as Ezlynn Ortega. Police also identified her 1-year-old brother as Jerricho Ortega.
Ezlynn was found with skull fractures inside an Upland apartment on Tuesday and later died.
MORE: Upland PD: Mother to face charges after daughter dies, son thrown off balcony
Jerricho was found outside that apartment building after apparently being thrown off the two-story structure allegedly by his mother, Tierra Ortega.
Tierra Ortega, 24, then apparently jumped head first from her own second-story balcony onto the ground as police were arriving on scene, authorities said.
Witness Monica McHaskell said it all started with an "unusual cry" from a baby. Then she saw Ortega throw Jerricho off the balcony and then jumped right after.
"She was already up there. She threw a ball, I didn't know what type of ball it was, but she threw a ball. Then she chucked the baby. As soon as I saw that, I ran out of the house, ran and picked him up and ran back in the house," she said.
Police said McHaskell held onto Jerricho and didn't let go until officers arrived.
Both children were airlifted to an area hospital. Ezlynn died from her injuries, and Jerricho sustained a broken foot and was listed in stable condition.
Tierra Ortega, who is expected to face murder charges, is set to appear in court on Thursday. Police are still trying to determine a motive.
Upland police will also hold fundraisers for Ezlynn's funeral expenses. The first will be on March 6, where people bring a flyer to a MOD Pizza location and 30 percent of the proceeds will be donated to the family.