(CNN) Lily wasn't moving when rescuers found her, hanging upside down in her mother's smashed car. It had flipped over into a frigid Utah river half a day before, and the baby was still strapped in her seat.

As Officer Jared Warner dashed with the 18-month-old in his arms to an ambulance, she was barely alive. But she's doing better, her family said Monday.

"Her improvement is astounding. Right now she's watching 'Dora (the Explorer)' and singing '(The) Wheels on the Bus' with Grandpa. She is smiling and laughing for family members. We're blown away by Lily's progress and so grateful to her rescuers," they said.

Lily's mother, Lynn Jennifer Groesbeck, died in the crash that had landed their car on its roof in the Spanish Fork River. She was 25 years old.

Lucky fishing trip

Lily might have died unseen with her mother, had a man not gone fishing in that particular spot Saturday. The angler waded into the river around noon, then noticed the car wheels-up in the water.

"Where the car was, you couldn't see it from the roadway," police Lt. Cory Slaymaker said.

The car was partly submerged in the frigid Spanish Fork River.

The fisherman called emergency dispatch.

Police and fire rescuers arrived and sprang into the water. The rest of what happened is now a frenetic blur for Officer Bryan DeWitt. "I don't remember doing anything but just doing it," he said.

The water was so cold that, when the rescue was over, seven of the men involved had to be treated for hypothermia.

They heaved the car onto its side and saw Groesbeck in the driver's seat. It was clear to them that she was dead.

Then they noticed the baby. "She was definitely unconscious and not responsive," Warner said.

But she was still alive, and rescuers were delighted to see it.

Trapped 14 hours

She was still strapped into her seat, where she may have been for 14 hours, if the wreck occurred when police believe it did.

The night before, a man living nearby heard a loud crash and stepped outside to check, but he saw nothing unusual, police said, and dismissed it.

That was at about 10:30 p.m. on Friday. Groesbeck's car had probably just struck a bridge embankment.

"The driver's side tire went up the cement barrier on the south end of the bridge, launching this woman and her baby into the freezing water," Slaymaker told CNN affiliate KUTV.

The angler came along around 12:30 p.m. on Saturday, 14 hours later.

How did baby survive?

Groesbeck's death is tragic, but the survival of baby Lily is a story filled with hope.

Above all, said CNN senior medical correspondent Elizabeth Cohen, the biggest factor was the car seat.

Lily was in the proper car seat and the seat appears to have been properly attached, Cohen said.

Even though the child was trapped and upside down, her body remained in the seat and above the frigid water. Doctors say that such low temperatures are dangerous, but would be even more so if the baby were wet.

Dry cold temperatures are more survivable than wet cold temperatures, Cohen said.

Another factor may have been that the girl, as a toddler, has baby fat. If she had been an infant, it might have been a different story, Cohen said.

It's a reminder that the human body is tough. Lily apparently survived for 14 hours in extreme cold, without food or drink.

Rescuers heard a voice

A mystery arose from the rescue: The police officers who entered the water say they heard a voice calling for help.

The mother was dead, but the officers said that they heard an adult's voice calling to them.

"The four of us heard a distinct voice coming from the car," Warner told CNN. "To me, it didn't sound like a child's voice."

The voice gave the rescuers a surge of adrenaline needed to push the vehicle upright, he said.

The mother was dead. The child was unconscious, but her eyelids were fluttering, and the rescuers knew she was alive, Warner said.

It's one of those things that doesn't have an explanation, he said about the voice.

"It felt like I could hear someone telling me, 'I need help,'" DeWitt told CNN affiliate KSL. "It was very surreal, something that I felt like I could hear."

Tyler Beddoes, a third officer at the scene, said the same.

"All I know is that it was there, we all heard it, and that just helped us to push us harder, like I say, and do what we could to rescue anyone inside the car," he told CNN's "Anderson Cooper 360."

No skid marks

The river is 10 feet deep in some places, police said, but the car hit a shallower spot. Images showed it only partly covered by water. Lily was not submerged, said Police Lt. Matt Johnson.

Firefighter Paul Tomadakis freed her from the wreck. He "grabbed the baby in my arm, raised its head up out of water, as I tried to release the seat belt," he said.

Then Warner took Lily into his arms.

"The child was passed to me and I ran up and climbed into the ambulance with the child," he said.

Police have no explanation for the wreck. No alcohol or drugs were in play, they told KUTV. There were no skid marks leading up to the impact.

A tow truck hauled Groesbeck's car out of the river, and it will be inspected for possible mechanical failures.