The frantic hunt for a young woman believed to be a threat to Columbine High School, which put hundreds of thousands of Colorado students and parents on edge, is over.

The body of Sol Pais, an 18-year-old from Florida who this week flew to Denver, obtained a pump-action shotgun, was found Wednesday at the base of a mountain 65km west of Columbine High school, the FBI said.

The FBI had described Pais as "armed" and "extremely dangerous" and harbouring an "infatuation" with the 1999 Columbine massacre. She was last seen in the foothills of Jefferson County, clad in camouflage pants, black boots and a black T-shirt.

Sol Pais, 18, was considered
Sol Pais, 18, was considered "extremely dangerous," the FBI said. Photo / AP

Classes at Columbine were canceled Wednesday. The state's eight largest school districts were also closed, impacting more than 400,000 students in a community forever defined by the April 20, 1999 school shooting that killed 12 students and one teacher.

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"It's unnerving. Parents I know are very, very worried about this," said Jim Earley, 50, the father of 13-year-old twins who attend Mandalay Middle School, located about a 20-minute drive north from Columbine High School in the sprawling Jefferson County School District. "But parents, especially in my generation, understand."

While children remain at home, law enforcement officials conducted a "massive manhunt" for Pais, the head of the Denver office of the FBI, Dean Phillips, said on Tuesday.

"She has made some concerning comments in the past," Phillips said. "She has expressed an infatuation with Columbine."

FBI agents were seen entering Pais' home in South Florida Tuesday night. A man who identified himself as her father told the Miami Herald of his daughter: "I think maybe she's got a mental problem." Pais's parents reported the teenager missing Monday night, after losing contact with her on Sunday.

Online postings and a journal that appear to have been written by Pais indicate that in the last year she's been grappling with a heightened sense of isolation and anger.

In an entry dated January 15, Pais - whose online handle is "Dissolved Girl" - hinted that she might carry out an attack. "My views and thoughts [are] becoming more extreme and solidified as time goes by," she wrote on her personal web site's diary. "[I] feel like a pot of scolding water on the verge of boiling over."

In late March, Pais wrote on the website of the National Gun Forum, asking how she could acquire a weapon in Colorado if she lived in another state.

Hello everybody," she wrote. "Florida resident here. I am planning a trip to Colorado in the next month or so and wanna buy a shotgun while I'm there and I was wondering what restrictions would apply for me? I've found a few private sellers I might want to purchase from... Thank you for reading, I appreciate your response!"

Several users chimed in with advice.

"First of all, thank you for taking the time to answer my question and in that extent -- I really do appreciate it," she wrote. "I have it all planned out, have friends coming with me who own guns and know what they're doing."

Pais's interest in Columbine is far from a new phenomenon to law enforcement agencies in Colorado. The 1999 attack has long been the subject of intense online interest to individuals known as "Columbiners," who obsess over investigation records, school blueprints and writings from the shooters made public.

Dozens of school shooters, including the killers at Sandy Hook Elementary School and Virginia Tech, are known to be among these obsessives. They publicly or privately worship the Columbine shooters, Eric Harris and Dylan Klebold, who killed 12 of their classmates and one of their teachers in 1999.

"People relate to them because they view Columbine as a case of the oppressed students rising up against their oppressors, the bullies," said Peter Langman, an expert on psychologist who studies school shooters . "That's a complete misunderstanding of what happened at Columbine - but it is a common view of it."

As the 20th anniversary of the attack on Columbine has approached, the Jefferson County Public School District has seen an increase in threats and concerning messages, which often come in the form of emails to the school or phone calls to the 24-hour dispatch center run by the district's security team.

More than 150 people per month have been apprehended in the Columbine parking lot, attempting to take photos of the school or get inside it.

While most of those threats and trespassers have been deemed harmless, the threats by Pais were considered far more serious.

John Nicoletti, a psychologist who works as a consultant to the Jefferson County Public Schools, said the accumulation of actions made by Pais - buying a plane ticket, taking the flight, purchasing a weapon - shows "somebody who is committed to carrying this out."

"What we want to avoid is saying we don't think this person means it," Nicoletti said. "Then, if they do do something, how do you explain that?"

WHERE TO GET HELP:

If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.

OR IF YOU NEED TO TALK TO SOMEONE ELSE:

0800 543 354 (0800 LIFELINE) or free text 4357 (HELP) (available 24/7)
https://www.lifeline.org.nz/services/suicide-crisis-helpline
YOUTHLINE: 0800 376 633
• NEED TO TALK? Free call or text 1737 (available 24/7)
KIDSLINE: 0800 543 754 (available 24/7)
WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757 or TEXT 4202