PASADENA, Calif. -

Police say they arrested two high school students suspected of planning a massacre at a school after investigators monitored their Internet activities.
    
South Pasadena police Chief Arthur Miller said Tuesday that school officials had heard about the plot and informed police, who determined the threat was credible.

The intended targets, three school  staff members and as many students as possible.

"They had a very real threat, they had a plan in mind that they were going to execute, then later on in our interviews with the suspects they more or less had confirmed what they had talked about cold heartedly," Miller said.
    
Police say the boys, ages 16 and 17, didn't have weapons but were researching automatic weapons and explosives, especially propane.

Students say its a chilling way to begin a new school year.  

"I really don't want to go to school on Thursday. It's a lot of anxiety to know that you're going to school where people planned to murder you," said Kelly Lake, a South Pasadena High School student.

The two teenage boys were arrested on charges of making threats and conspiracy. It was unclear when they planned to carry out their attack.
    
Miller said his officers saved lives by thwarting the attack in the Los Angeles suburb of about 25,000 people known for its high-quality schools.

The FBI has joined the investigation. Police have not named the boys.

School official say it's a reminder for parents and students to be vigilant and take every threat seriously. They urge anyone who sees something to say something.