A 12-year-old girl upset about being grounded by her mother fatally shot the woman in the face while she slept, police said.

Elvira Marion Walton, 48, was discovered in her bed early Sunday by her son. The 10-year-old boy called police around 1 a.m.

"Apparently the motive is because the daughter was upset that the mother disciplined her," police Sgt. Gary Kirkpatrick said in Monday editions of The Dallas Morning News.

The girl has been charged with murder. Police found a gun in the house, but investigators did not immediately say whether it was the weapon used in the shooting or who owned it.

An older daughter, Thanica Derrick, said her mother had been having trouble with the girl. Walton had six children and lived in a converted garage used as the family home.

"She is your average 12-year-old, hormones and everything," said Derrick, 22. "There's nothing that bad to make her do that to my mama. She had been breaking out of the house and not going to school."