Police are also investigating whether the mother had been drinking.
Police say the girl was riding in a car with her mother along Dexham Road just before 7 p.m.
The mother, 24-year-old Cynthia Del Angel, rear-ended a pickup truck at the intersection of Lakeview Parkway and Dexham Road as she was heading toward Garland, according to police.
The driver of the truck pulled into a nearby gas station to exchange information and thought Del Angel did, too.
Instead, police say Del Angel took off at a high rate of speed southbound on Dexham Road. She hit some DART tracks a quarter-mile away, lost control and skidded into a tree.
Her 5-year-old daughter , Layla, who was sitting in the back seat with only an adult seatbelt on was fatally injured. She died at Lake Pointe Medical Center in Rowlett.
Del Angel was taken to a nearby hospital with serious injuries.
Police suspect that Del Angel was intoxicated, but they say they will not charge her with anything until they have finished their investigation, which includes a toxicology report.
Police say that reports that the crash was related to a domestic dispute are untrue.