A student leaves Columbine High School late Tuesday in Littleton, Colo. The school was closed Wednesday while authorities looked for a woman they said presented a credible threat ahead of the 20th anniversary of the mass shooting there.

A student leaves Columbine High School late Tuesday in Littleton, Colo. The school was closed Wednesday while authorities looked for a woman they said presented a credible threat ahead of the 20th anniversary of the mass shooting there.