A young Florida woman who travelled to Colorado and bought a shotgun for what authorities feared would be a Columbine-inspired attack just days ahead of the 20th anniversary was found dead Wednesday in an apparent suicide after a nearly 24-hour manhunt.

Jefferson County Sheriff Jeff Shrader said 18-year-old Sol Pais was discovered by the FBI with an apparent self-inflicted gunshot wound.

The manhunt had led to the closing of Denver-area schools as a precaution.

During the manhunt, the FBI said Pais was “infatuated” with Columbine and threatened violence ahead of Saturday’s anniversary of the attack that killed 13 people at Columbine High School in 1999.

The FBI described her “extremely dangerous”.

The Miami Beach high school student flew to Colorado on Monday night and bought a pump-action shotgun and ammunition, authorities said.

Agents had focused the search around the base of Mount Evans, a popular recreational area about 60 miles southwest of Denver.

All classes and extracurricular activities for about half a million students were cancelled as a precaution, though sheriff’s spokesman Mike Taplin said the young woman’s threats were general and not specific to any school.

“We’re used to threats, frankly, at Columbine,” said John McDonald, executive director of security for the Jefferson County school system, when the manhunt was over.

“This one felt different. It was different. It certainly had our attention.

Earlier, Dean Phillips, agent in charge of the FBI in Denver, had said: “This has become a massive manhunt and every law enforcement agency is participating and helping in this effort.”

Authorities said Pais had last been seen not far from Columbine, in the Jefferson County foothills outside Denver, in a black T-shirt, camouflage trousers and black boots.

The alert also said police who came into contact with her should detain her and evaluate her mental health.

Colorado governor Jared Polis said federal, state and local law enforcement were “dedicating all of their resources to locate this dangerous individual”.

“We know that there is a lot of anxiety right now in Colorado,” Polis said in a statement.

Because of the threat, Columbine and more than 20 other schools outside Denver locked their doors for nearly three hours on Tuesday and some cancelled evening activities or moved them inside.

Columbine lockdownStudents leave Columbine High School following a lockdown (David Zalubowski/AP)

Pais’s parents last saw her on Sunday and reported her missing to Florida authorities on Monday night, police in Surfside, Florida, said.

Adam Charni, a Miami Beach High School pupil, said Pais dressed in black and kept mostly to herself. He said he was “baffled” to learn she was the person authorities in Colorado were searching for.

Another classmate, 17-year-old junior Drew Burnstine, said Pais was a quiet, smart student who sat alone in class and “never caused problems or indicated that she wanted to harm anyone”.

Two teenage gunmen attacked Columbine on April 20, 1999, killing 12 classmates and a teacher before taking their own lives.