Story highlights Witnesses scramble to help after a woman drives her minivan into Atlantic Ocean

Three children are rescued and are in the custody of the state

The woman is being held for a mental evaluation

She was stopped by police earlier in the day, but they couldn't find cause to detain her

Tim Tesseneer was driving along Daytona Beach on Tuesday with his wife when they noticed the minivan driving through shallow water as it headed south.

That was unusual enough. But then they heard the screams. It was two children crying and waving for help out one of the rear windows.

Tesseneer threw the car in park and raced over to see whether he could get the children out of the minivan.

One child was screaming, Tesseneer recalled Wednesday to CNN's Piers Morgan. "Please help us, our mom is trying to kill us."

The other child he could see was wrestling a woman for the steering wheel.

But the woman just kept saying, "We're OK. We're OK. We're OK," as another man joined Tesseneer trying to get the driver to stop.

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Then she mashed the accelerator, turned left and shot into the cold, heavy surf of the Atlantic Ocean touching the shore of the Florida city.

It was now a battle against the pull of the water, and the second man, Stacy Robinson, opened a door and pulled out the two panicked children.

There was a good chance if he and Tesseneer hadn't been there, the children, ages 10 and 9, would have drowned inside the van as it pitched in the water, officials said.

"I have siblings their age, so it was like the big brother came out in me," Robinson said.

But there was one more child, a 3-year-old girl strapped in a car seat. A lifeguard dove in through a front window and unbuckled the child and handed her to another lifeguard. The vehicle was bobbing in water about 3 feet deep.

The mother just walked away, Tesseneer said, silent with a strange, almost "possessed" look on her face.

A day later she was at a hospital in Daytona Beach awaiting a mental evaluation, Volusia County Sheriff Ben Johnson said. Her children, including her unborn baby, were fine.

No charges had been filed, Johnson said, as the investigation into why she drove into the ocean was just beginning.

CNN is not identifying the children or the woman, who was stopped earlier in the day after her sister called police to let them know how concerned she was for her sibling's mental state.

According to a Daytona Beach Police report, the woman was stopped by an officer, who questioned her. In the back seat the kids sat calmly with smiles. The woman told an officer that she feared for her safety and was worried her former husband would harm them.

But the officer was concerned that she might have a mental illness. He asked a detective who was there to talk to her while other officers called the sister back.

In the report, the officer writes of discussing the woman's behavior with the detective.

They came to the same conclusion; she couldn't be held under a Florida law that allows for detention of people believed to be impaired by mental illness and who possibly pose a risk of harm.

They told her she could go; one officer followed her for a short time.

Almost two hours later, she was driving on the beach, outside the traffic lane, drawing the attention of beach patrol and bystanders like Robinson and Tesseneer, on vacation from North Carolina.

Sheriff Johnson said the children are now in the state's care. They had been interviewed by detectives from his agency, which was taking the lead because it has more investigators than other departments.

The woman had yet to speak with detectives.

Johnson said his team will try to determine whether this was a criminal act or one that resulted from a medical issue.

No one in his department knows how long the woman had been in town or why she was visiting from South Carolina. She did have family in the area, the sheriff said.