South Pasadena police on Tuesday said two teens arrested this week had developed a "huge plan" to carry out a mass school shooting in which they wanted to kill "as many people as possible."
South Pasadena Police Chief Art Miller told reporters at a news conference the two boys, ages 16 and 17, had researched weaponry, explosives and methods for disarming people.
The teens, who have not been identified, "very cold-heartedly" discussed their plans with each other online.
"As they put it, they just wanted to kill as many people as possible," Miller said.
The teens, who were arrested Monday, also told investigators they were willing to die in a shootout with police, Miller added.
Police launched the around-the-clock investigation Thursday based on a tip about a possible school shooting plot that was relayed to administrators at South Pasadena High School, he said.
The investigation included reviewing social media posts and "Internet surveillance," Miller said. On Monday, police served search warrants at the boys' home. No weapons were found at the home, and there was no target date, he added.
There was some "pretty frightening information" gleaned from the computers and interviews with the suspects, Miller said.
"As they were planning it ... they had a very specific plan on how they were going to carry out their sick mission," he said.
News of the alleged plot shocked many in this close-knit community, including Deb McCurdy, who will be president of the parent-teacher association next year.
"We're stunned, but relieved the systems in place worked," she said.
South Pasadena High student Hanna Crowley, 16, said she "could not have imagined" a mass shooting could have happened at the campus.
"You would think someone from the outside would do this, but never a student," the incoming senior said.
Police planned to turnover the results of their investigation so far to county prosecutors for consideration of possible charges.
The teens were being held on suspicion of conspiracy and criminal threats, Miller said.
The lack of weapons, meanwhile, appeared to temper some of the fear.
"It's very alarming and very sad, the good thing is, it was detected," said Graham Witherall, whose daughter attends South Pasadena High. "I'm curious about whether this was a couple of kids fantasizing, or if this was a real plot."
Two Santa Clarita Valley teens were also arrested over the weekend on suspicion of making threats of deadly school violence on social media. But Los Angeles County Sheriff's officials determined the boys, ages 13 and 15, had made the threats as pranks that spiraled out of control.
Their identities also were not released.
South Pasadena, McCurdy said, is a safe, small community, and a mass killing would seem unlikely here.