NEVADA- A 12-year-old boy who killed a teacher and wounded two fellow students at his Nevada middle school likely brought the gun from home, raising questions about whether the parents could face liability.

A day after the shooting, police were searching for reasons why the boy, among the youngest school shooters in U.S. history, opened fire with a Ruger 9mm semi-automatic handgun in his schoolyard just before the opening bell on Monday.

A popular math teacher who tried to intervene to stop the shooting was killed in the gunfire, and two other 12-year-olds were wounded before the boy, a seventh grader, took his own life.

Police have so far declined to identify the young shooter, citing a need to protect his grieving parents, who were cooperating with the investigation. Police have said they believe he obtained the gun from his home.

Police are continuing to investigate exactly how the boy got the gun, and have interviewed the parents of the shooter. The shooting was the latest in a string of U.S. gun rampages in recent years, including one in December at a Connecticut elementary school that killed 20 students and six adults.