A six-year-old girl from South Phoenix, Ariz., succumbed to her injuries on Thursday after her brother, 8, took her on a joyride in their mother’s car on Wednesday night and crashed it into a pole, police told media on Thursday.

The boy suffered minor injuries to his ankle, but the girl -- who police believe was not wearing a seatbelt -- died in a Phoenix hospital on Thursday morning, reports said.

"This is just a horrific event," Steve Martos, an officer with the Phoenix police department, told Reuters. "The mom puts them to bed, tucks them in for the night and then wakes up this morning and finds that her six-year-old is dead. That's just a tragedy."

The children were reported missing at around 10.25 p.m. local time (1.25 a.m. EDT) on Wednesday night after their mother rang up emergency services reporting that her two children and the family’s Hyundai Elantra were missing. The children's mother had put them to bed an hour earlier, and had not realized that the kids were missing until the woman's mother and the kids' grandmother -- who lives nearby and was driving home -- noticed that her daughter's front door was open, and alerted her daughter.

According to the Reuters report, the children had driven five miles and were on their way back home when the accident occurred. The boy appeared to know how to drive a car as witnesses told NBC News that he was stopping at red lights and obeying other traffic laws.

CNN reported that emergency services started getting numerous frantic calls reporting that a car had struck a fence at 10.15 p.m. and within 15 minutes, they had located the car and were trying to pull the vehicle over when it hit a pole.

"This was one of those situations when the officers believed they were trying to rescue kids from a possible kidnapper," Martos told CNN. "When they ran up to the vehicle, they found an 8-year-old in the driver's seat, and a 6-year-old in the passenger seat, severely injured."

The identity of the mother and children were not immediately released, and Martos told CNN that no criminal complaint is expected to be filed.