Two boys planned shooting at a suburban Los Angeles high school was cut short as investigators acted after receiving a tip unravelling their plot, ultimately arresting them. The two boys planned shooting reportedly involved killing three staffers at their Southern California high school, then gunning down as many students as possible, said police.

According to ABC News, school officials learned of the two boys planned shooting on Thursday. South Pasadena Police Chief Arthur Miller said that after the school notified detectives, police began their surveillance of the online activities of the 16- and 17-year-old boys.

Apparently, the boys were already trying to get weapons as they did not have them yet.

According to South Pasadena police Sgt. Brian Solinsky on Monday, the boys had been under constant surveillance ever since the tip from the school district. Though Solinsky did not elaborate further on the two boys planned shooting, he said they were "very specific" and included named targets.

CBS Local reports South Pasadena Board of Education Superintendent Geoff Yantz stating Monday that the administration was informed of a "credible threat of potential school violence", which in turn led to contacting police immediately.

According to Yantz, there was already no threat as the school prepared to open for the fall semester, since the students had been arrested.

He said, "Two students involved in the matter were arrested and remain in custody. The police have the situation under control and there is currently no threat to students or employees."

He added, "Psychologists and counselors will be available at the high school to provide support to our students and employees. The School Board and I appreciate the immediate response by the South Pasadena Police Department, and school will begin on as planned."

According to the Associated Press, South Pasadena police Sgt. Solinsky said in a statement, "This is a prime example of school officials recognizing suspicious behaviour. It was this information that helped prevent a horrific tragedy."

The two boys planned shooting had no date of attack, however their online messaging had specific names of staffers they planned to kill and threats towards other students, said South Pasadena Police Chief Miller.

Police also reportedly found evidence that the boys were also researching automatic firearms, rifles, submachine guns, handguns, knives, explosives especially propane and tactical techniques.

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

He added of the two boys planned shooting, "Three or four days' worth of surveillance on the Internet indicated that they had a very real threat. They had a plan in mind that they were going to execute."

South Pasadena police Sgt. Solinsky said that eventually, they had enough evidence gathered to serve warrants serve warrants at the boys' homes on Monday. Police even had to break into one of the boys' houses as he resisted and tried to run when police tried to take him into custody.

According to ABC News, the boys' names had not been released because of their ages. They were arrested on charges of making threats and conspiracy and are being held at a juvenile facility.

The AP reports that the boys were supposedly about to begin their senior year at South Pasadena High School, eight miles northeast of downtown Los Angeles, in a community consisting of about 25,000 people.

Their first day of classes is scheduled Thursday, and police initially planned to make the arrests before then. Police planned to have a larger presence than usual on campus for the school's first day of classes.

Solinsky said that since the threat of the two boys planned shooting first emerged; detectives had been hard at work around the clock, monitoring the boys' activities.

ABC News reports that the FBI had also joined in the investigation to help search for evidence on the boys' computers at home.

Relatives of both boys had also been questioned about the two boys planned shooting, and they found out one boy with a relative with a gun he might be able to get. Fortunately, when police contacted that relative, the weapon was found secured.

No further information or evidence on the investigation of the two boys planned shooting has been elaborated by police.

Meanwhile, students have since expressed their relief.

Student David Yun told KCAL9's Rachel Kim, "I'm really shocked."

Another student said, "I would never imagine something like that happening at our school."

However, reactions of school cheerleaders Zoe Bullard and Leah Schexnayder, both 16, regarding the two boys planned shooting, are the opposite. Both were frightened by the news, even though the boys were arrested.

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

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

Two boys planned shooting investigation details will likely be revealed Tuesday morning as the police chief had planned for a news conference then to reveal the details.