Tampa Police Chief Brian Dugan thought witnesses - who watched in horror as a mother dragged her screaming 4-year-old daughter into the murky waters of the Hillsborough River on Thursday - were mistaken.
Cops said Denson stole a vehicle from a local auto shop and drove it to the river, where she parked and then waded into the waters with the small child.
The child, later identified as Je'Hyrah Daniels, was rushed to St. Joe's Hospital, where she was pronounced dead.
"You can imagine we do have a few witnesses that called us, and they're kind of shaken up a bit", Tampa Police Chief Brian Dugan said at the scene, "and so it's a insane scene that we are still unraveling numerous pieces".
"It's a completely tragic event", Dugan said.
"We have a few witnesses that called us", he said.
She said she didn't see a child, but thought something wasn't right.
After ditching the Nissan Altima, which she allegedly stole from an auto shop, Denson is accused of wading halfway across the body of water, before jettisoning her daughter.
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Denson had recently sought help for her daughter who she said was on the autism spectrum.
Neighbor Vicki Walker witnessed the whole thing from the balcony of her condo.
Police said they responded to a report of a child being thrown from a bridge at 4:30 p.m., and a dive team member discovered an unconscious child submerged about 75 feet off shore.
WFLA reported that Denson then allegedly grabbed her daughter with both arms and pulled her tight against her chest as she walked deeper into the river and let the girl go.
Denson is also facing charges of child abuse and grand theft auto. Witnesses saw her do it, investigators said.
"It's really sad. I don't know the circumstances, I don't know what's going on in their lives, but it's killing me."
In a similar 2015 case, police say a man threw his 5-year-old daughter to her death off the Skyway Bridge in St. Petersburg as an officer tried to stop him.