SANTEE, Calif. A grinning teenage gunman opened fire on fellow students at a high school where he was often taunted on Monday, killing two and injuring 13 others in the latest act of violence to terrorize an American school.

The shooting just before 9:30 a.m. pst (12:30 p.m. EST/1730 GMT) at Santana High School in this suburb northeast of San Diego first created scenes of disbelief as students thought a cap gun was going off and then scenes of mass panic when they realized it was a real gun and started fleeing for their lives.

The shooter -- a 15-year-old ninth grader -- opened fire in a hallway after loading his gun in the boys' rest room. He apparently fired indiscriminately at anyone in sight, hitting two students fatally.

The boy had apparently told about 20 people over the weekend of his plans but no one believed him or reported him to authorities, although a relative of one of his friends said he patted him down and looked in his backpack before he left for school.

San Diego County Sheriff William Kolender said that authorities had confirmed that two students were killed and at least 13 other people injured. The alleged shooter was in custody, and will be prosecuted as an adult.

"There are two juveniles deceased, there are 15 injured including them. The suspect is in custody," Kolender told a news conference.

"We do not know the motivation. We do not know the motive at this point," he added.

The shooting was the latest of more than a dozen incidents of gun violence to have claimed casualties in American schools in recent years, including multiple killings in Oregon, Arkansas, Kentucky and Colorado.

The deadliest of these incidents to date was the April 1999 shooting spree at columbine high school in Littleton, Colo., where two teenage boys killed a teacher and 12 fellow students before taking their own lives. Twenty-three other people were injured in that rampage.

The alleged shooter at Santana was reported to often have been taunted by others but was "able to shrug it off."

Friends said the alleged gunman talked over the weekend of going to school with a gun but then when questioned by adults he claimed to be only kidding and said his guns were locked up at home.

Chris Reynolds, an adult who knew the alleged shooter, said the boy began talking about the violent plan over the weekend -- but that few people took it seriously. "Everybody kind of thought he was joking around," Reynolds told a television reporter from local station KGTV. "If somebody did die over there and stuff, that's going to be haunting me for a long time."

Reynolds said the suspect was often the butt of jokes at school, but that he usually replied with a joke and smile.

Witnesses said the boy was smiling as he opened fire with a handgun on his fellow students, who were milling through the hallways during a change in morning class periods.

One boy reportedly died at the scene of the attack, a small courtyard at the high school. The other, a 15-year-old boy, died later at a local hospital.