ROWLETT — A mother says a tiny GPS tracking device may have saved her 15-year-old daughter's life.

The teen has Asperger's, an autism spectrum disorder.

And Monday, the device may have stopped a sexual assault from going further. The mother bought it just last month because her daughter has a tendency to wander off.

"I can tell if she stops, I can tell if she turns around, I can see everywhere she goes," the mother said.

Police say when her daughter went on one of her regular walks Monday, a man got her into his vehicle on Magnolia Street.

Then, just after 4 p.m. Monday, the device sent a text that the teen was in an unregistered location, about two miles away, at Springfield Park. It allowed her mother to listen in, and what she heard is every parent's nightmare.

"I heard the assault while it was happening," said the mother. "I knew exactly what was happening."

Police say a man was sexually assaulting the woman's daughter in a car, parked in a gravel area at the busy Springfield Park in Rowlett.

So the mother, phone in hand, drove to find her daughter.

"He found the device on her waistband, told her to take it off," said the mother. "She said she can't, ‘My mom is the only one that can do it.' And I think it freaked him out, spooked him."

That's when the man drove off from the park, dropping her daughter off near her home. Her mother wasn't far behind.  

"She had been dropped off on a corner, she was running, and I found her there," said the mother.

She says her daughter is doing well, considering the circumstances. She underwent a medical examination shortly after the crime.

Investigators interviewed the teen victim Thursday, giving police a better picture of the man as they continue their search. They say the tracking device, which saves location data, is giving them a rare edge on the investigation.

"That gives us a timeline, that also gives us a location to look for evidence," said Rowlett Police Lt. David Nabors.

The service is called AngelSense. It provides a GPS tracking device that can be clipped on to your child's clothing with a strong magnet that is difficult to remove. It texts parents when their child leaves the house or is in a location where they shouldn't be. It shows their location on a map, and includes what direction they are going and how fast they are going.

It also allows parents to listen in on their child's surroundings.

"The company refers to your child as the angel," said the victim's mother.

And it's helping the mother keep her own angel safe.

Police describe the suspect as a man in his 20s with a mustache and a tattoo of a circle with points coming out of it on his right forearm.

He was wearing a white T-shirt, dark shorts, and Nike sandals at the time. His car is a white mid-size sedan that smelled like cigarettes and had two car seats and a booster seat in the back.

Police are asking anyone with information to call 972-412-6200.

If you were at the park Monday and noticed anything suspicious, you're also asked to call police.

If you would like to find out more about the device, go here.