The police chief of South Pasadena, Calif., said Tuesday that two high school students have acknowledged planning a "sick mission" to shoot three staff members and random students at South Pasadena High School.
"They wanted to kill as many people as possible," Police Chief Art Miller told reporters.
The two boys -- ages 16 and 17 -- were arrested Monday on preliminary charges of conspiracy and criminal threats and that other charges are under consideration.
Police Sgt. Robert Bartl said the pair planned to carry out their attack for personal reasons, the Pasadena Star-News reports.
"They may have been crossed by one of the students or teachers, maybe by how the teacher taught or a grade (one of the suspects) received," Bartl said. "It was definitely directed toward people they weren't happy with."
Police said earlier that one of the suspects was arrested without incident at his home, while the second allegedly tried to flee arresting officers, who forced entry into his house.
Miller said the parents of the two teens were "stunned" but very cooperative.
Police learned of the alleged plot from school officials who were tipped by an unidentified member of the community.
Sgt. Brian Solinsky said in a statement that school officials became aware of suspicious behavior by the pair last week and contacted law enforcement authorities. "It was this information that helped prevent a horrific tragedy," he said.
Miller, the police chief, said the teens' computers, which were seized through a search warrant, showed the teens discussing the plot "coldheartedly."
He said the suspects had researched the Internet on how to obtain and use automatic weapons, handguns, knives, bulletproof vests and explosives.
He said the teens, in interviews with police, had largely confirmed the plot as outlined in their social media chats.
Miller said the alleged plot was only at the "very beginning" and that police had not found any weapons.
He also said that the pair had not set a target date but "had a very, very specific plan how they were going to carry out their sick mission."
School officials said the high school will open on Thursday as scheduled, although with heightened security.