Columbine High School was placed on lockout Monday (Sept. 22) because of a threat.

Officials tweeted that classes are running as scheduled, but no one is allowed to enter or leave.

Jeffco Public Schools has a Standard Response Protocol, lockout, lockdown, evacuate and shelter.

Lockout is called when there is a threat or hazard outside the school building.

We'll update this story as more information becomes available.

[The Denver Channel]