Broward County Sheriff Ken Jenne says the mother has been interviewed and that her comments "put a whole new light on this story." He declined to elaborate before a news conference later in the day.
The baby, whose umbilical cord was still attached, survived with minor injuries and was hospitalized in serious condition Friday, reports Joan Murray of CBS station WFOR-TV.
"He weighs 8-pounds, 2-ounces, and I have to tell you, I saw him, he looks beautiful, he's a little fighter, full of life," Veda Coleman-Wright, of the Broward County sheriff's office, told WFOR-TV.
The boy was thrown out of the car alongside a busy street in North Lauderdale, 13 miles northwest of Fort Lauderdale, on Thursday afternoon, Sheriff Ken Jenne said. The car then sped away.
A woman stopped and found the baby, believed to be less than an hour old, inside a small plastic bag. She scooped him up and took him to a nearby sheriff's office, and he was transferred to Broward General Medical Center.
"She stopped her vehicle to rescue what she thought would be a cat or a kitten," Jenne said. "She got out of the car opened it up took a plastic bag off the head of the individual and discovered the newborn."
The good Samaritan was "very distraught, very upset," Jenne said. The woman's name was not released.
The baby's rescuer told investigators she observed a man and woman
arguing inside the vehicle. But she saw only the back of their heads, Jenne said, and could not provide a good description.