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SOUTH PASADENA, Calif. (AP) — Two teenage boys have been arrested 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 Monday.

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 behavior," 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.

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