A 2-year-old remained in critical condition Sunday after being shot by his 4-year-old brother, who may not have known the difference between a real and a toy gun, police said.

Police Sgt. Cameron Grysen (search) said the boys had been arguing at their home in southwest Houston (search) on Saturday afternoon when the 2-year-old threw a toy at his brother.

The older boy got the loaded gun from a purse in his mother's bedroom and shot his brother once in the temple, Grysen said.

He was being treated at Ben Taub Hospital (search).

The mother told police she had the .32-caliber automatic to protect her family because of recent neighborhood burglaries. She said Saturday was the one day that she did not secure the weapon. She could face criminal charges.

Child Protective Services spokeswoman Estella Olguin said CPS had never been called to the home before. The agency has placed the older boy with a grandparent.

Authorities said the 4-year-old didn't seem to understand what he had done.

"He's wondering where his brother is, and when his brother's coming back," Grysen said.