Loretta Wilson dismissed her daughter's concerns and said it was just the hum of the car's cooling fans.
But throughout the night on May 30, carbon monoxide fumes built up in the house, killing Amber and her friend Caitlin Brondolo, 12, who was at the Wilsons for a sleepover.
Wilson was charged Thursday with two counts of manslaughter by culpable negligence for the death of the Loggers Run Middle sixth-graders.
According to an arrest report, deputies said Wilson, 45, left her 2006 Ford Escape running in the garage of her home in the Loggers Run community west of
Boca Raton before watching a movie and going to bed that night.
Wilson woke up the next day to find the two girls dead. The
Palm Beach County Medical Examiner's Office determined the girls died from carbon monoxide poisoning.
"It is more careless than it is on purpose," said Sheriff's Office spokeswoman Teri Barbera, of Wilson's second-degree felony charges. "Working with the State Attorney's Office, it was determined she should be charged."
Detectives tested the SUV and said they found no apparent defects. The car was "in good condition and the vehicle was operating as it was designed," according to the arrest report.
During the tests, the cooling fans did not remain running after the ignition was turned off, as Wilson claimed.
She told investigators that she would regularly turn off the car's engine, leave the keys in the ignition and then close the garage door.
Wilson, too, was affected by the odorless, colorless fumes. She was hospitalized briefly but made a full recovery.
The Brondolo family could not be reached Thursday despite a message left on their home phone.