SOUTH PASADENA, Calif. — Investigators acting on a tip unraveled an alleged plot to carry out a mass shooting at a suburban Los Angeles high school, arresting a pair of students who planned to target three school staffers and kill as many people as possible, police said Tuesday.
School officials learned of the plot on Thursday and notified detectives, who began watching the 16- and 17-year-old boys and monitoring their online activity, South Pasadena Police Chief Arthur Miller said.
The pair did not have a date for an attack or weapons, but their online messaging included the names of three staffers to target and threats to randomly kill students, Miller said.
They were also researching automatic firearms, handguns, knives, explosives, and tactical techniques, he said.
‘‘Three or four days’ worth of surveillance on the Internet indicated that they had a very real threat,’’ he said. ‘‘They had a plan in mind that they were going to execute.’’
Miller said police interviewed the boys and realized how cold-hearted the plot was. He provided no specifics, and officials at South Pasadena High School did not disclose details of the initial tip.
Classes start again Thursday in the quaint San Gabriel Valley suburb known for its good schools and community involvement in education. Police plan to have a larger presence than usual on campus for the first day of school.