Two teenage boys who were allegedly planning to carry out a mass school shooting were arrested Monday, just days before they were set to start their senior year at their Southern California high school.

Police say the teens, ages 16 and 17, made specific plans to kill three staffers at the school and open fire on as many students as possible in the Los Angeles suburb of South Pasadena.

After the police were informed of their deadly plans by the school district last Thursday, the boys were placed under heavy surveillance, said South Pasadena police Chief Art Miller. Miller said a member of the community notified officials at the school district about the plot, reports CBS News.

"This is a prime example of school officials recognizing suspicious behavior," Sgt. Brian Solinsky said in a statement, according to the Associated Press. "It was this information that helped prevent a horrific tragedy."

The teens were arrested after authorities gathered enough evidence to serve search warrants and retain their computers. Some of the evidence revealed that the boys were researching rifles, submachine guns, bombs and different explosives, like propane, said officials.

"They were researching weapons and how to fire and assemble them," police Sgt. Robert Bartl told the Pasadena Star-News.

Although there did not appear to be a specific date for the attack, Miller noted that the teens had a "very, very specific plan of how they were going to carry out their sick mission."

The boys "were making a huge plan of a school massacre," he said.

Authorities have yet to disclose a motive in the case.

The parents of both students have been cooperating in the ongoing investigation, Miller said.

He also stated that the teens had spoken "very cold-heartedly" of the plot with investigators.

First day of classes in the South Pasadena district is scheduled to begin on Thursday.