Two teenage boys were arrested Monday after making specific plans to kill three staffers then gun down as many students as possible at their Southern California high school, police said.
The boys, who did not yet have weapons but were trying to get them, had been under constant surveillance since the school district informed police of their plans last Thursday, South Pasadena police Sgt. Brian Solinsky said.
He would not elaborate on the plans or what form they took, but Solinsky said they were "very specific" and included named targets.
"This is a prime example of school officials recognizing suspicious behaviour," Solinsky said in a statement. "It was this information that helped prevent a horrific tragedy."
Police found evidence that the boys were researching rifles, submachine guns, bombs and other explosives, especially propane, police said.
"They were researching weapons and how to fire and assemble them," police Sgt. Robert Bartl told the Pasadena Star-News.
Enough evidence was gathered to serve warrants at the boys' homes Monday, Solinsky said.
One of them resisted, and police had to break into his house to take him into custody as he tried to run, Solinsky said.
The boys were both about to begin their senior year at South Pasadena High School in the community of about 25,000 people eight miles northeast of downtown Los Angeles.
The school's first day of classes is scheduled for Thursday, and police had hoped to make the arrests before then, though they found no evidence of a date for a planned attack, Bartl told the Star-News.
Detectives had been working around the clock and monitoring the boys since the threat first emerged, Solinsky said. Relatives of both boys had been questioned, he said, but would not elaborate further on the investigation or the evidence.
The police chief plans a news conference Tuesday morning to reveal more details.
The arrests came the day after another Los Angeles County boy was arrested on suspicion of posting online threats to shoot students at local schools, though sheriff's officials acknowledged that those threats were intended as pranks.