Chico Quinones and his wife were inside their Milwaukee home watching TV late Saturday night when he heard a crash.
"I heard like two trains colliding, that is what it sounded like," he said.
When he rushed outside, he saw that two SUVs and a car had collided in the street across from his home in the city's South Side neighborhood.
Quinones had a security camera pointed at the street, and caught the entire incident on video.
In the video, a Cadillac appeared to disregard a stop sign, hit an oncoming Ford Explorer, which then hit another SUV. The three cars were piled up in the middle of the street.
The video shows Quinones and others in the neighborhood rushing out of their homes to see what had happened. The neighbors approached the vehicles and started helping its occupants.
As Quinones drew closer to the vehicles, he heard the faint sounds of a young girl crying. The sounds were coming from the Ford Explorer that was lying on its side.
"And I could hear her cry and yelling for her mother and father, so I could not just leave the baby there," Quinones, who has 14 grandchildren, told " Good Morning America" in an interview that aired on the show today. "To hear a baby cry like that, it breaks your heart."
The video shows Quinones approaching the SUV.
He saw that the girl's mother had been knocked out. She was lying curled up in a ball near the front seat, while the girl - a toddler - was still strapped into her car seat.
He crawled into the vehicle and started working to free the girl right away. He recalled that there was a "very strong smell of gasoline," and said he feared the SUV could explode at any moment.
"I was telling her in Spanish, like I called her sweetheart, 'Mamita, don't worry. I am not going to leave here without you,'" he said.
Everyone involved in the crash escaped with minor injuries.
The driver and passenger in the car that caused the accident were seen on tape walking away from the scene. The driver was later found and charged with a hit and run.
ABC News' Benjamin Krolowitz and Mosheh Gains contributed to this report.