The girl only a suffered a broken jaw, and her family said she is expected to make a full recovery.
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Firefighter Ryan Ciampoli's dashboard camera recorded the harrowing moment on Wednesday when the door swings open with the girl clinging to it. In the video, she is seen quickly falling onto the road and appears immobile. Then, the bus drives away.
VIDEO: Volunteer firefighter saves 4-year-old who flew out of bus
The video shows Ciampoli approaching the girl and gently scooping her up.
"Once the adrenaline and the shock kicked in her little body, she started kicking and screaming and you know: 'where's my mommy?' and things like that, so stuff like that is really heartbreaking," Ciampoli said.
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Ciampoli later told reporters he normally wouldn't have moved her due to the nature of her fall, but he was concerned because the accident happened in the middle of a state highway.
Police have not said if the girl opened the door, or if it wasn't locked.
ABC7 News contributed to this story.