Denver-area public schools will be closed on Wednesday as authorities search for a young Florida woman who flew to the city and bought a gun after becoming "infatuated" with the mass shooting at Columbine High School.
The FBI said Sol Pais, 18, is "considered to be extremely dangerous" and "made threats to commit an act of violence in the Denver metropolitan area" just days before the 20th anniversary of the attack that killed 13 people.
All schools in the Denver area were urged to tighten security because the threat was deemed "credible and general", said Patricia Billinger, a spokesperson for the Colorado Department of Public Safety. Columbine and more than 20 other schools outside Denver lock their doors for nearly three hours onTuesday afternoon before Wednesday's complete closures were announced.
The Jefferson County Sheriff's Office and the FBI say Pais travelled to Colorado from Miami on Monday night and bought a pump-action shotgun and ammunition.
Denver Public Schools said that all facilities and programmes will be closed on Wednesday, and there will be no afternoon activities or athletic competitions. The district said the decision to close campuses was in collaboration with other Denver metro-area school districts due to the ongoing safety concern.
On Tuesday, some schools released their students after additional security was called in and cancelled evening activities or moved them inside.
"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 on Tuesday night.
The FBI's Rocky Mountain Safe Streets Task Force issued a notice on Tuesday describing Pais as "infatuated with (the) Columbine school shooting".
Sheriff's spokesperson Mike Taplin said the threats she made were general and not specific to any school.
The Denver Post reported that a call to a phone number listed for Pais' parents in Surfside, Florida, was interrupted by a man who identified himself as an FBI agent and said he was interviewing them.
Surfside Police Sgt. Marian Cruz confirmed that her parents last saw her on Sunday and reported her missing on Monday. The Miami Herald and WTVJ are reporting that neighbours say the teen is a senior at Miami Beach High School.
The Associated Press left messages at two numbers listed for Pais' relatives in Florida, while another number was disconnected.
Two teenage gunmen attacked Columbine on April 20, 1999, killing 12 classmates and a teacher.