Two 16-year-old girls have been arrested in Douglas County on suspicion of plotting to kill classmates and staff members at Mountain Vista High School.

The sheriff’s office said authorities learned of the threats Saturday. In addition to conspiracy to commit first-degree murder, officials said the girls could face other charges.

Investigators called the threats against the Highlands Ranch school credible, but they did not elaborate.

“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.”

Bronner said one girl was arrested Monday night, and she declined to say when the other was taken into custody.

Officials were alerted to the threat through a “Text-A-Tip” program in the Douglas County School District. Students were not in classes Tuesday because of a snow day.

“I didn’t hear anything about it (until Tuesday),” said Thomas Johnson, a 16-year-old junior at Mountain Vista. “I am very glad for this snow day we got today. 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.”

Johnson said his high school has great students, and he was shocked to hear about the plot.

“We take these types of threats extremely seriously and will take all efforts necessary to ensure the safety of our schools, students and teachers,” Sheriff Tony Spurlock said in a statement. “We thank the Douglas County School District for all their assistance during the investigation.”

Authorities did not immediately release further details, citing their ongoing investigation. The girls’ names have been withheld.

County prosecutors are working with the sheriff’s office on the case but declined to comment because the case involves juveniles.

A spokeswoman for the district attorney’s office said she is barred from releasing information unless either girl is charged as an adult.

Court records have been sealed.

“At this time, we believe the situation has been contained and the threat disrupted,” the sheriff’s office said.

“We want you to know that many of the safety systems we have in place worked well,” Liz Fagen, superintendent of the school district, said in a letter to parents.

Fagen said the systems helped identify the plot and prevent it from being carried out.

“These two students have been apprehended and face serious felony charges,” Michael Weaver, principal of the 2,200-student high school, said in a letter to parents.

Chris Grollnek, a Dallas-based police expert whose studies include incidents of school violence, said the Mountain Vista plot stands out because two-girl attack teams are historically rare.

“There have been active (attacker) events with girls in school, but there’s never been a dynamic duo, if you will,” said Grollnek, a former SWAT officer.

The sheriff’s statement came on the same day that an e-mailed threat shut down the nation’s largest school district, closing 900 public schools and 187 charter schools in Los Angeles.

Also Tuesday, Mesa County authorities arrested a 13-year-old girl who allegedly posted a photo on Face book of a gun and threatened to harm her middle school classmates in Palisade.

Dozens of Mountain Vista students took to social media to share news of the threats and comfort each other. Many were bewildered and expressed thanks that the plot didn’t come to fruition.

“Love you all Vista,” one girl posted on Twitter.

Chris Torres, the parent of two boys who attend Mountain Vista, called news of the threats a “pretty big shock.” He learned of the plot through the letters from Fagen and Weaver.

“There always seems to be security or police presence around,” he said of the school’s campus. “To me, it just seems like a safe and secure facility. I’m thankful that nothing actually went down.”

Jesse Paul: 303-954-1733, jpaul@denverpost.com or @JesseAPaul

To help:

Anyone with information on the case is asked to call the sheriff’s office at 303-660-7500. Non-emergent information should be phoned into 303-660-7579.