Two Southern California male teenagers were being held in juvenile hall today after an investigation by authorities using social media discovered the two were in the early stages of a plot to allegedly commit a mass shooting at South Pasadena High School.
"It was a very viable threat what they were plotting," said South Pasadena Police Chief Art Miller. "They were making a huge plan of a school massacre. … During our interviews with the suspects, they, more or less, confirmed what they had talked about, very cold-heartedly."
Miller said the suspects, whom police did not identify, were 16 and 17. They were arrested Monday on suspicion of conspiracy and criminal threats.
The investigation started Thursday after an anonymous person informed the school's staff about the alleged plot. The staff then contacted police about a possible school shooting in the works.
Miller said detectives watched the teens' conversations on social media and eventually were able to get a search warrant Monday.
Miller said that three staff members were targeted by name as were "random" students.
"As they put it, they just wanted to kill as many people as possible," Miller said.
On Monday afternoon, officers went to two homes in South Pasadena and removed two computers. Miller said a search of the teens' computers found the students had researched weapons as well as how to make bombs.
He said the students were in the beginning stages of their plan and were also looking online about tactical training.
He said they told detectives they were prepared to die.
"[They were] very monotone, very matter-of-fact, and when you are talking about killing people, shooting them in the head and to be so calm about it. It's very chilling," he said. "It's very frightening."
No weapons were found in the houses. Miller did not reveal a motive or a target date and said that the parents were stunned.
"I can't emphasize enough how that one phone call to us got the ball rolling," he said. "This press conference today would have been a much different one, had we not acted."
School is set to start Thursday.
ABC News' Cecilia Vega contributed to this story.