The shotgun a 16-year-old used to kill three children and a teacher at Silver Rail Elementary School in Bend, Ore., last week was a birthday gift from his mother, according to sources cited by CBS News.

The pump-action 12-gauge shotgun was bought legally in August and given to him for recreational use, the sources said, according to CBS. A law enforcement official also said last week that the gun was bought by his mother, KCRA reported, citing AP. No one other than the shooter has been accused of a crime.

The parents of 5-year-old Rhoam Summer, one of the children killed, have hired lawyers to look into the school's safety plans and how the teen got the gun, KCRA reported. "Of interest is how the shooter, a minor, accessed his weapon," said Tim Williams, a Bend lawyer for the family. The lawyers are also asking whether the school had an active-shooter plan in place.

The other children killed were 8 and 11, police said. A 7-year-old girl and a 54-year-old teacher were wounded, CBS reported. Nine people were in the classroom at the time, police said Monday.

Police said the shooter first fired at a passing pickup truck, then shot through the glass of a set of double doors to get into the school, CBS reported. He killed all four victims in the first classroom inside. A bystander who saw the first shots followed him into the school and shot and killed him. The shooter was a former student at the school, police said.

Bend-La Pine Schools Superintendent Steven Cook said the district has "invested heavily" in cameras, door locks and fencing. "We practice lockdowns with students and staff twice a year at every school," he said, KCRA reported.

Rhoam's parents said he "wanted to go to the pumpkin patch, learn to ride horses, learn to play an instrument, and be a bunny for Halloween."

No motive has been released.

What we know: Sources say the gun was a legal purchase given to the teen as a gift, and a family is investigating.
What we don't: Why the shooter attacked the school and whether anyone else will face charges.