SOUTH PASADENA, Calif. (AP) — Police said they prevented a "horrific tragedy" by arresting two teenage boys who plotted to kill three high school staffers then gun down as many students as possible in a quaint Los Angeles suburb.

The boys, who were trying to get weapons, 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. The boys' names haven't been released. Police planned a news conference Tuesday to provide more information.

"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 the boys were looking for information on submachine guns, rifles, bombs and other explosives, especially propane.

"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.

Police had to break into one boy's house as he resisted and they took 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, a well-regarded school. The leafy San Gabriel Valley suburb of about 25,000 people 6 miles from downtown Los Angeles is known for its high-quality schools, which drive up the price of homes. Small two-bedroom bungalows can easily top $700,000.

Police aimed to make the arrests before the first day of class Thursday, 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. Relatives of both boys had been questioned, Solinsky said, but would not elaborate further.

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.