SOUTH PASADENA, Calif. (AP) — Investigators acting on a tip unraveled a plot to carry out a massacre 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 didn't 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."

The names of the teens have not been released because of their ages. Police expect to present their case to the district attorney later in the day.

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 quality schools and community involvement in education. Police plan to have a larger presence than usual on campus for the first day of school.

School cheerleaders Zoe Bullard and Leah Schexnayder, both 16, said they were frightened by news of the arrests.

"I don't even want to go to school on Thursday, because what if something happens?" Schexnayder said.

"What if they missed a person?" Bullard asked about the police investigation.

Miller said his officers saved lives by thwarting the attack in the town of about 25,000 people about 6 miles from downtown Los Angeles.

"They were making a huge, a huge plan of a school massacre," Miller said. "They just wanted to kill as many people as possible."