Colorado police and the FBI are hunting an 18-year-old woman they say is “armed” and “extremely dangerous”, and who prompted a lockdown at Columbine High School just days before the 20th anniversary of the deadly shooting there.

That school and 20 others were ordered to undergo the special security measures after police received what they called a “credible threat”.

Police have identified the suspect as 18-year-old Sol Pais.

The Jefferson County Sheriff's office and the FBI said the teenager travelled to Colorado on Monday night and made threats, but provided not further details. The law enforcement agencies she had last been seen in the foothills west of Denver, where some of the schools put on lockdown are located.

Students left classes on time, but after-school activities were cancelled at Columbine in Littleton, Colorado.

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“She is armed and considered to be extremely dangerous,” the sheriff's said in a statement. “She is a white female, 18 years old, approximately 5'5“ in height, with brown hair. She was last seen wearing a black t-shirt, camouflage pants, and black boots.”

The Denver Post said police considered Ms Pais to be “infatuated” with the shooting at Columbine, in which 13 students and staff were killed by pupils Eric Harris and Dylan Klebold, who took their own lives after police responded.

The mass shooting of 20 April 1999 appeared to make an inflection point in relation to gun violence and mass shootings at schools, triggering a series of copycat incidents.

Among the mass school shootings that followed were Virginia Tech, Sandy Hook, and Parkland.

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For the generation of students that was educated in the 20 years since Columbine, lockdowns and security drills have become a constant of school life. The AP pointed out security firms have forged a multibillion-dollar industry, introducing surveillance video, panic buttons and upgraded doors and locks. Police changed their strategies for responding to a gunman intent only on killing.