Two South Pasadena teens arrested for plotting mass shootings in their school have been formally charged, the Los Angeles County District Attorney's Office said Wednesday.

The boys, 16 and 17, students of the South Pasadena High School, face one count of making criminal threats. Additional charges may be filed after more evidence is found against them. Their names have not been disclosed because of their ages.

The duo denied the charges during a hearing in Pasadena Juvenile Court, according to the Los Angeles County. As previously reported the teens conspired to kill three staff members and several students at the South Pasadena High School with firearms. The officials came to know about the conspiracy last Thursday after the school administrators tipped them off about the plot. Following  this, the police followed their behavior and arrested them Monday after they collected enough evidence about their mass-shooting plans.

According to the police, the teens were researching about automatic firearms, handguns, knives, explosives and tactical techniques. Prosecutors said the teens shared their plan with another boy and on Saturday threatened to kill him, reports the Associated Press.

"There was no target date but they had a very, very specific plan," police chief Arthur Miller said at a press conference, according to Reuters.

Step-father of one of the suspects issued a statement saying, "He had no intention of going to the school and actually harming the people that he loves. My wife and I would like to apologize to the whole community, every student, every parent, every faculty that we do not condone any kind of behavior," reports abc7.com.

The South Pasadena High School is witnessing heavy security and the school is expected to resume classes Thursday.