CASTLE ROCK — Authorities in Douglas County on Wednesday credited a text-message tip with thwarting an imminent plot to kill students and staff members this week at Mountain Vista High School.

Two 16-year-old girls remain in custody one day after investigators announced their arrest in the case.

If it hadn’t been for an anonymous “Text-A-Tip” alerting deputies to the teens’ alleged conspiracy, their plans might have been successful, Douglas County Sheriff Tony Spurlock said.

“I think the text message and the information we obtained through our investigation saved lives, for sure, given the severity of the situation,” Spurlock said.

In the wake of the 1999 attack on Columbine High School, Colorado officials have fielded hundreds of tips on threats through different initiatives urging teens to speak up.

Douglas County authorities have received 231 alerts through their Text-A-Tip program in this school year alone.

“Whoever texted the tip — THANK YOU!!” a senior at Mountain Vista tweeted Tuesday.

Law enforcement say tipsters give them a unique view into parts of students’ school life they don’t typically have access to, including social media interactions and peer-on-peer conflicts hidden beneath the surface.

“We have no idea what’s going on in the schools all the time,” Spurlock said.

Douglas County School District superintendent Liz Fagen said at a Wednesday news conference that most threat tips are not found to be credible. Each alert, however, is seriously scrutinized.

She called Text-A-Tip an important layer in the district’s overall safety strategy, which is full of redundancies.

“It’s an important part of allowing students and others to feel safe,” Fagen said.

The program is specific to Douglas County’s school district, but students statewide have for years been using the similar Safe2Tell network established in 2004 and run by the state attorney general’s office.

Safe2Tell received nearly 3,200 reports — including for bullying, suicide prevention and drug and alcohol abuse — in the 2013-14 school year. That’s up more than 3,000 percent in the past decade.

In November, 600 tips came in, representing a 99 percent increase over the same month last year.

In Douglas County this year, there have been 85 Safe2Tell tips.

“There’s a need to expand,” said Jose Esquibel, who oversees the program as director of the attorney general’s newly formed Office of Community Engagement. “We have a small staff. As we have increased numbers of reporting, we need more (people.)”

Esquibel said there have been 412 reports of planned attacks since the initiative began in 2004.

Safe2Tell in the 2013-14 school year received tips on 54 planned campus attacks. In the 2012-13 school year, tips on 60 planned school attacks were received.

In August, the initiative launched a mobile application that has since become the most popular way for tips to be submitted. The app led authorities in Cañon City in October to uncover a massive sexting scandal involving more than 100 students.

John McDonald, executive director of security and emergency management for Jefferson County Public Schools, told his board this year that the data for the district are “concerning” because they increasingly involve younger students.

“Students are becoming more violent at a younger age,” he said. “They see it and read about it so much more. Social media drives a lot of behavior today. There are a lot of students who don’t think there’s a way out.”

Mountain Vista students returned to classes Wednesday as a long line of cars carried teens to school in a biting cold. The building in Highlands Ranch was shut Tuesday because of deep snow — a break some students theorized on social media could have saved lives if the plot hadn’t been stopped.

District officials said a crisis team was on standby for students in need and that deputies were increasing patrols to ensure school security.

“We know the gravity of what has happened in the past,” Fagen said of previous school attacks and the need to prevent them in Douglas County.

Prosecutors have not filed formal charges against the girls suspected in the murder plot. They face a long list of possible allegations, authorities say, including conspiracy to commit first-degree murder.

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