Two 16-year-old girls were taken into custody after allegedly conspiring to kill their classmates and staff members at their suburban Denver high school on Tuesday.

The teens, who have not been identified, were arrested Monday night, the day before they planned to execute an attack at the Mountain Vista High School in the affluent suburb of Highlands Ranch, reports The Associated Press. The Douglas County Sheriff's Office said it was first made aware of the threat on Saturday after someone alerted authorities by texting a tip line.

According to CBS Denver, the young women were taken into custody after Douglas County Sheriff's deputies determined that they posed a credible threat.

"The Douglas County Sheriff's Office has investigated and confirmed a credible threat to MVHS from two female, MVHS students who planned to cause harm to our staff and students at MVHS this week," the Douglas County School District said in a letter sent to parents of students within the Douglas County School District.

Sheriff's officials added that the teens could face felony charges for conspiracy to commit first-degree murder.

"It was a threat against students," Sgt. Lori Bronner, spokeswoman for the sheriff's office, told The Denver Post. "They took an overt -- a substantial -- step in their plans. Luckily, we were able to find out about it and stop it."

"I think it's pretty crazy," said student Carlie Curran, who attends the school, to ABC 7 Denver. "You don't know what could happen, and it's our school. You're supposed to be in a safe place there."

Officials added that both suspects are being held in "secured facilities."

"We take these types of threats extremely seriously and will take all efforts necessary to ensure the safety of our schools, students, and teachers," Douglas County Sheriff Tony Spurlock said.

Classes were cancelled at the school on Tuesday due to snow.