Denver’s South High School was on lockout for the third day in a row Friday following tips about a student who made a threat linked to the school. Police were looking for the student, who the principal described in a message to parents as a runaway.

“Tonight [the Denver Public Schools] safety chief spoke with the Denver police chief’s office and we were assured that finding this student is a top priority within their department,” Principal Bobby Thomas said Thursday night in a message to parents. “Denver police have also sent out a statewide alert so that all law enforcement agencies are actively looking for him and can arrest the student based on an outstanding warrant.”

Thomas continued, “Please understand that while learning is vital, we want students and staff to feel supported and safe while this investigation continues. I know the events of this week have been very challenging, and I’m trying to share as much as I can.”

Advanced Placement exams scheduled for Friday were canceled, and the school had mental health counselors available to speak with any students experiencing anxiety, Thomas said. The school continued to have “an increased presence” of Denver police and district safety officers on Friday, he said, and students who stayed home would have their absences excused.

The South High lockout began on Wednesday, just one day after a fatal shooting at a suburban Denver high school.

A lockout is less serious than a lockdown. In a lockout, schools lock their exterior doors, and students and staff members remain inside. Classes continue as usual.

Thomas wrote to parents on Wednesday that the lockout was “standard protocol” while Denver police and Denver Public Schools safety officers investigated two tips to the anonymous Safe2Tell hotline about a threat connected to the school.

“After several hours, police shared with us that they do not believe there is a credible threat to our school,” the Wednesday message said. “We kept our school in lockout until the end of the school day out of an abundance of caution.”

But early Thursday morning, parents got another message saying the lockout would continue.

“We monitored the situation at South High School overnight and the school will remain on lockout with a closed campus today,” the message said.

It did not provide an explanation as to why, or whether the credibility of the threat had changed.

Social media posts suggest many South High students stayed home Thursday. One teacher tweeted that fewer than 40 percent of students showed up to school. South High is Denver’s second-biggest school, serving nearly 1,600 students in grades nine through 12.