All schools in the Denver area were urged to tighten security because the threat was deemed "credible and general," said Patricia Billinger, a spokeswoman for the Colorado Department of Public Safety. Columbine and more than 20 other schools outside Denver lock their doors for nearly three hours Tuesday afternoon before Wednesday's complete closures were announced.
The Jefferson County Sheriff's Office and the FBI say Pais traveled to Colorado from Miami on Monday night and bought a pump-action shotgun and ammunition.
"We always have heightened awareness close to high-profile anniversaries like this," Billinger said.
Authorities said Pais was last seen near Columbine -- in the Jefferson County foothills outside Denver -- wearing a black T-shirt, camouflage pants and black boots. They appealed for anyone seeing her, to call an FBI tip line at 303 630-6227, and said she is too dangerous to be approached by civilians. The alert also said police who come into contact with her should detain her and evaluate her mental health.
"This has become a massive manhunt ... and every law enforcement agency is participating and helping in this effort," Dean Phillips, special agent in charge of the FBI in Denver, said late Tuesday night.
The Columbine school shooting had two teenage gunmen attack Columbine on April 20, 1999, killing 12 classmates and a teacher.