It is unclear how long she had been dead when she was found. Jefferson County Sheriff Jeff Shrader said officials believe she acted alone, but said they are combing through her social media and personal blog — where she appears to have published several disconcerting posts — to look for possible accomplices.

Pais traveled to Colorado this week from her home in Florida. Upon her arrival, she legally purchased a pump-action shotgun at a gun shop near Columbine High School and then, seemingly, disappeared.

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The threats Pais made triggered a lockout at Jefferson County Public Schools Tuesday, and on Wednesday, the state’s eight largest school districts were closed. More than 400,000 students were impacted, adding to what was already a stressful week for the community: the 20th anniversary of the 1999 shooting at Columbine High School is on Saturday.

“We are used to threats, frankly, at Columbine. This one felt different,” said John McDonald, the head of safety and security for the school district. “It was different.”

With the crisis over, schools will reopen on Thursday. Memorial events for the anniversary, including an alumni reunion at the high school, will go on as planned.

Pais’s parents reported the teenager missing Monday night, after losing contact with her on Sunday. She is not known to have a criminal record; The Miami-Dade Police Department said Wednesday that it had searched incident reports going back 18 years and found none under Pais’s name.

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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 “dissolvedgirl” — 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!”

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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.

Pais purchased three one-way plane tickets from Miami to Denver, for April 15, 16 and 17, said Dean Phillips, the special agent in charge of the Denver FBI office, in a Wednesday press briefing. She boarded a plane on Monday.

Pais landed in Colorado later that day, Phillips said, and went looking for a weapon.

The owner of Colorado Gun Broker, a shop two miles away from the high school, confirmed on Wednesday that his store sold Pais the shotgun two days earlier.

“We had no reason to suspect she was a threat to either herself or anyone else,” said the shop owner, Josh Rayburn, in a Facebook post.

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Rayburn said that Pais passed a “full background check” and cleared the FBI’s warning system.

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“We are very sorry to hear of the outcome in this situation,” he said. “It is never good when someone loses their life.”

Authorities believe Pais then traveled to the foothills of Mount Evans, where an Uber driver dropped her off. The car’s driver told the FBI that Pais hadn’t said anything suspicious during the ride, Phillips said at the briefing.

That was the last time she was seen.

After she got out of the car, Pais traveled about a half mile down a foot trail near the base of the mountain, said Rick Albers, the sheriff of Clear Creek County, about 30 minutes west of Denver. She then veered from the trail to walk uphill through a snow-packed forest of pine trees. When Albers and other law enforcement officers found her body, at around 10:50 local time Wednesday morning, she was dead from a self-inflicted gun shot. She was wearing a black shirt, a multi-colored plaid jacket, and camouflage pants. Her weapon was found close to her body.

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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 the 1999 attack and other school shootings. “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.

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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?”

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That fear reverberated across the country, through Pais’s high school in Miami Beach. Students were worried that she’d return to Florida, and described an anxious atmosphere and a campus with more police officers than usual. Pais, they said, was a quiet student who took Advanced Placement classes.

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“She kind of stood out because of the way she dressed, goth style," said Dimitri Odige, a freshman at her school. "Kind of like she was from another era, like, maybe the 90s.”

After news broke that Pais was no longer at large, parents in the Colorado community felt only a fleeting sense of relief. The anniversary of the 1999 shooting was still two days away.

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“We’re doing what moms do, we grieve, we cry and watch the news conferences and try to figure out how we are going to talk to our kids about what happened,” said Tina Galterio, 46, who has a first grader in the Jefferson County schools.

Galterio graduated from Columbine High School in 1992 and she wept as she recounted how she’s taken her child to the Columbine Memorial to pay respects “on a regular basis.”

After the Parkland shooting last year, Galterio purchased a bulletproof backpack for her first grader. Now, she will have to decide whether or not she feels ready to send her son back to school tomorrow.

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Julie Tate, Mark Berman, Lori Rozsa, and Jennifer Jenkins contributed to this report.