DENVER — The Columbine High School principal who spent years educating students and schools about lessons learned from the 1999 massacre will no longer talk to visitors about the tragedy, unless they have been cleared by school security.

Frank DeAngelis said he's changing his open-door policy after learning that a Utah high school student who visited him in December to talk about the Columbine shootings was implicated this week in a plot to bomb a high school near Salt Lake City.

DeAngelis said the 16-year-old, whose name is being withheld because he is a minor, told him that he was writing for a school newspaper. The youth showed him a student ID and wanted details about the shooting and steps taken to protect students.

"He asked the same questions I get from many callers and visitors asking about the shooting," DeAngelis told The Associated Press. "This was definitely a wake-up call. This is the first time this has happened."

DeAngelis was the principal at the suburban Denver high school when students Eric Harris, 18, and Dylan Klebold, 17, opened fire on April 20, 1999, killing 12 classmates and a teacher and wounding 26 others before killing themselves in the school's library. DeAngelis shepherded a group of about 20 students to safety that day.

The massacre prompted schools nationwide to install metal detectors, fences, ID badges, dress codes, security patrols and surveillance cameras.

After the shootings, DeAngelis struggled to balance safety and students' rights. The school installed a comprehensive security camera system and keyless entry cards for faculty. He also vowed to stay in his post until all students who were in kindergarten the year of the bloodshed had graduated.

DeAngelis said Friday he often gets interview requests from students researching the shootings and tries to accommodate them.

He said that in 2006, he notified authorities in Hillsborough, N.C., that he had received an email from a 19-year-old man obsessed with the Columbine massacre — but that it was too late to prevent an attack.

The man, Alvaro Castillo, was convicted in 2009 of killing his father and opening fire outside his former high school. Two students suffered minor wounds when they were shot in the Orange High School parking lot.

DeAngelis also said he offered advice to officials at Virginia Tech after student Seung Hui-Cho killed 32 people and himself in April 2007.