DENVER – After a massive one-day manhunt, a woman allegedly infatuated with the Columbine mass shooting was found dead Wednesday of a self-inflicted gunshot wound.

Sol Pais, who made "credible threats" against Denver-area schools four days before the 20th anniversary of the Columbine High School shooting, fatally shot herself, Jefferson County Sheriff Jeff Shrader said.

The 18-year-old South Florida woman flew Monday night from Miami to Colorado and immediately bought a pump-action shotgun and ammunition at a gun shop in the Littleton area, not far from Columbine High School, Shrader said.

Dean Phillips, special agent in charge for the FBI in Denver, said Pais was found dead in the Mount Evans area near the Echo Mountain ski resort west of Denver on Wednesday morning around 10:30 a.m. to 10:40 a.m. He said she was last spotted there Monday after being dropped off by a ride-share vehicle.

It's too early to say when she killed herself, he said, adding that the investigation is ongoing to ensure there were no additional participants or sign of foul play. Her body awaits an autopsy. He said at this point it looks as if she was alone and took her own life with the weapon she had bought.

"It was a long night," Phillips said, pointing to command posts that went around the clock and crediting a robust effort among multiple agencies, states and levels of government. "It was an incredible effort in the Denver metro area – I can tell you."

Pais' actions and comments relayed to law enforcement from people she had spoken to prompted officials to close schools across the Denver area – including Columbine. They are set to reopen Thursday.

Her death capped an anxious day in the Denver area and a search launched the previous night after Pais, dressed in camouflage pants and black boots, disappeared in the foothills west of Denver. Officials from Florida and Colorado coordinated together for the investigation after her "concerning comments" about Columbine drew their attention.

Raising alarm further: Pais purchased three one-way airplane tickets from Miami to Denver for flights that departed on consecutive days, Monday, Tuesday and Wednesday.

"Very unusual activity that caused us great concern that obviously we worked very quickly to identify and address," Phillips said.

The school closures affected more than 500,000 students in Colorado, extending north toward the Wyoming border and south toward Colorado Springs.

"We are relieved that the threat to our schools and our community is no longer present," said Jefferson County Public Schools Superintendent Jason Glass. "While this threat has been the most significant, it's not the only challenge facing our schools and our community in these days leading up to the Columbine anniversary.

"I appreciate our community's patience as we continue to navigate these difficult issues."

John McDonald, executive director of safety and security for the school district, said administrators are used to threats at Columbine, but this one “felt different, was different and certainly had our attention.”

“We know that Columbine continues to attract people from around the world. And if I have any message: We’re not a place to come visit if you’re not a student. If you don’t have business there, we’re not a tourist attraction and we’re not a place for you to come and be an inspiration.”

Pais was a senior at Miami Beach Senior High School. She was said to be reserved and enrolled in Advancement Placement classes.

From Surfside, Florida, where Pais lived with her family, Police Chief Julio Yero said, "I would like to express the family’s grief for this situation. They are grateful that no one else is hurt.”

He commended the FBI, law enforcement in Colorado and even the Pais family for their work in finding the teen.

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“The collaboration probably prevented a tragedy," Yero said. "This family contributed greatly to this investigation from the very onset. They provided us valuable information that led us to Colorado.”

Pais appears to have purchased the gun legally. Representatives from the Colorado gun shop that sold the firearm to Pais said the woman passed a background check from both the Federal Bureau of Investigation system and Colorado Bureau of Investigation.

“We had no reason to suspect she was a threat to either herself or anyone else,” Josh Rayburn, owner of Colorado Gun Broker, wrote. “We are very sorry to hear of the outcome in this situation. It is never good when someone loses their life. We are praying for her family. And are very thankful this situation did not escalate into a public tragedy.”

It appears Pais also had an interest in guns.

One person identified as “Sol Pais” ran an online blog that included pictures of guns and journal entries filled with angst and other disturbing messages.

"The purpose of this site is for me to give insight into the thoughts I rarely, if ever, share with others, while remaining somewhat anonymous. Everything from journal entries to my personal interests – I want to leave a record of myself before I, well..." the blog says.

In the "About me" section, the individual wrote, “I am the face of loneliness and misery.”

USA TODAY was not able to independently confirm that the blog belonged to Pais. Phillips said the FBI has been "combing all manner of her social media" and that the online journal is among the leads they are looking at.

"We're following up on every lead to see if there was any other accomplices or other matters that we need to follow up on," he said.

A person with the same screen name, "dissolvedgirl," posted on a National Gun Forum message board about living in Miami and traveling to Colorado to buy a gun. Law enforcement did not say whether the person was Pais.

Pais' parents reported her missing Monday night, according to the Surfside Police Department.

The FBI’s Rocky Mountain Safe Streets Task Force issued a notice Tuesday that said police who come into contact with Pais should detain her and evaluate her mental health.

The threats she made were general and not specific to any Colorado school, officials said.

The Denver, Douglas County and Cherry Creek school districts called off Wednesday classes, activities and athletics at all schools late Tuesday night, citing "safety concerns related to a credible threat against schools in the area."

The Jefferson County School District, which includes Columbine, also called off classes for Wednesday.

The news came of Pais' death came as a shock at the school Pais had attended, Miami Beach Senior High School.

A teacher found Andrea Luis,18, Crying in a bathroom after she learned of the death of Pais. The teacher sent Luis to the counselors office where there were three other students coping with the news.

“She just does not match the profile of a school shooter,” Luis said. “She was not violent or aggressive and was the type to always give you a compliment and was very apologetic. She was also very quiet,” Luis said, holding back tears.

Although Luis said she was not very close to Pais, she remembers working alongside her as a volunteer at a book fair and said she looked up to her because she seemed very confident.

“She had this striking presence and I loved her style. She was one of the very few people who had the same taste in music as me,” she said.

Luis was in disbelief when she heard the FBI was looking for Pais.

Students described Pais as a quiet girl who was very good at art. Lesley Gent, a teacher and chair of the Art Department at the high school, remembers seeing Pais in the hallways.

“She was very artistic,” Gent said. “Up until last quarter, she had excellent grades. This past quarter, they just dropped.”

The school made an announcement Wednesday morning when news was out that something happened with a student but that it happened off campus, Luis said.

Miami-Dade County Public Schools will be sending the mental health department to the school along with crisis counselors to assist students and staff, Jackie Calzadilla, a spokeswoman with the school district said.

During the search for Pais, the sheriff's office described her as "last seen wearing a black T-shirt, camouflage pants, and black boots."

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April 20 is an emotional date for many Denver-area parents and students because it's the anniversary of the Columbine shooting in which two students killed 12 of their classmates and a coach before killing themselves. There's typically an uptick in threats made to schools around that date every year.

Garrison reported from Boston. Rodriguez reported from Florida.

Contributing: Alan Gomez and Nick Penzenstadler, USA TODAY; The Associated Press