Colorado girls arrested for plot to kill students and staff at their high school

Two teenage girls took "substantial steps" in carrying out a murder plot at their high school before being foiled on Sunday, CBS Denver reports.

The girls, who have not been named because they are minors, were arrested and being held on no bail. They had planned to commit the crimes at Mountain Vista High School, in Denver. Both 16-year-olds have been charged with conspiring to commit first-degree murder, the Denver Post reports.

The girls are being held at separate facilities. At a Thursday court hearing for one of them, the judge ordered they not be allowed to have contact with each other. The other girl will undergo a mental evaluation.

"It was a threat against students," Douglas County Sheriff's Sgt. Lori Bronner told the Post. "They took an overt — a substantial — step in their plans. Luckily, we were able to find out about it and stop it."

Authorities learned of the plot through the Douglas County School District's program, "Text-A-Tip."

Schools were closed because of snow on Monday.

"I am very glad for this snow day we got today," junior Thomas Johnson told the Post. "I am very glad that my friends and our Vista student body is safe today. Who knows what could have happened if we had school today."

Authorities haven't released information about weapons the girls planned to use or any other details, citing an ongoing investigation.

"At this time, we believe the situation has been contained and the threat disrupted," the Sheriff's department told the Post.

The plot is one of several frightening events at schools as students are preparing to end the semester and go on winter break. This week, the Los Angeles Unified School District shuttered all its schools and told its 643,000 students to stay home after receiving a threat of violence. The threat turned out to be a hoax.