For weeks, the 18-year-old had been planning this trip, a pilgrimage from her home in South Florida to the Denver suburb where the modern era of school shootings had begun. She’d been studying that day and had become “infatuated” with the massacre, authorities said. Now she posed a significant threat to the school, its 1,700 students and the entire community.

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But instead of hurting others, Pais would end up running from the FBI — and killing herself with the shotgun she purchased.

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Officials said Pais was found dead in a remote mountain area Wednesday morning after a nearly 20-hour manhunt that shut down the state’s largest school districts and left hundreds of thousands of students and parents wondering just how close they had come to another attack on their community.

“This was the real deal,” said John McDonald, chief of security for the Jefferson County School District, in an interview with The Washington Post. “She was making significant statements to friends and social media. You add in the fact that she goes directly from the airport to the gun store . . . these are the red flags we always talk about.”

He learned about Pais on Tuesday morning, on a day the school district was already responding to a bomb threat at a middle school and receiving warnings about a potential shooting at another high school, McDonald said.

The Columbine community sees a rise in menacing messages and behavior every April, when the date of the 1999 attack approaches, but this year, with such a significant anniversary, the threats have been relentless. More than 150 people were apprehended in the Columbine parking lot last month, attempting to take photos of the school or get inside it.

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Most are deemed harmless. It was clear to authorities that Pais, who took Advanced Placement classes at Miami Beach Senior High School and had no criminal record, was probably not. The teenager’s parents reported her 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 she spent months struggling with an increasing sense of isolation and anger.

In a posting dated Jan. 15, she wrote — under the handle “dissolvedgirl” — “My views and thoughts [are] becoming more extreme and solidified as time goes by,” on her personal website’s diary. “[I] feel like a pot of scolding water on the verge of boiling over.”

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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.

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“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.

On Monday, law enforcement agencies in Florida were warned about Pais, who lived with her parents in an upscale Surfside neighborhood about two blocks from the beach. By Tuesday morning, that information was passed on to the FBI, which began retracing the young woman’s footsteps. They found that she had purchased three one-way tickets from Miami to Denver: one for Monday, one for Tuesday and one for Wednesday. Pais had been on the Monday flight. She was already in Colorado.

They learned about the ride she had taken to the gun shop and the shotgun she had purchased. She was last seen wearing a black shirt and camouflage pants. It was then that the Jefferson County School District put Columbine and other high schools on lockout, securing the exterior doors while continuing to hold classes. The FBI released Pais’s name and photo and began a massive manhunt.

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Parents in the area, already on edge because of the upcoming anniversary, began to panic. Jessey Smithwick, 40, said she paced up and down her street until she saw her fourth-grader’s bus.

She told her son, “Most people are good, and we won’t allow a few bad people to stop us from experiencing joy.” That night, she let him sleep beside her and locked the bedroom door.

Meanwhile, sixth-graders on a retreat in the mountains were being evacuated. Pais, authorities discovered, had traveled west. An Uber driver had taken her to the foothills of Mount Evans, about 40 miles from Columbine. The car’s driver said Pais didn’t mention anything suspicious during the ride, said Dean Phillips, the special agent in charge of the Denver FBI office, during a Wednesday news conference.

After Pais got out of the car, she walked about a half-mile down a trail near the base of the mountain, said Rick Albers, the sheriff of Clear Creek County. She then veered from the trail, uphill through a snow-packed forest of pine trees. As night fell, Pais still could not be found.

During the search, investigators were learning more about Pais’s fascination with Columbine — a mind-set that is far from a new phenomenon to Colorado law enforcement. The attack has long been the subject of intense online interest to people known as “Columbiners,” who obsess over investigation records, school blueprints and writings from the shooters.

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Dozens of school shooters, including the killers at Sandy Hook Elementary School and Virginia Tech, were among these obsessives. They publicly or privately worshiped the Columbine shooters, Eric Harris and Dylan Klebold, who killed 12 of their classmates and one of their teachers before taking their own lives.

“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.”

At 3:30 a.m., school leaders made the decision to call off school at eight of the state’s largest districts, affecting more than 400,000 students.

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McDonald said they were thinking about the safety of students: “What if she shows up at a school? We can protect the school. But if we engage in a gun battle on school property, even if we win, even if everything is perfect and the threat is neutralized, how do you tell parents, ‘Hey, your child was here when that happened?’ ”

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Across the region, parents and children were awake through the night, awaiting news of whether Pais had been found. McDonald received text after text from family members of students who died in 1999, offering him support.

Stephanie Sprenger, a Jefferson County parent and preschool teacher, learned her 12-year-old daughter had spent the early morning hours getting texts on her iPod with updates from her friends.

“Before I even woke up, she knew everything,” Sprenger said. “It’s terrifying to think my 12-year-old absorbed all the details before we talked to her.”

As Coloradans began their mornings glued once again to their TV screens, the FBI was zeroing in on Pais’s location. She was believed to be near the Echo Lake Lodge, a restaurant and campground nestled on a picturesque lake in the woods.

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At 10:50 a.m., Pais was found. By the time authorities reached her, she had died of what appeared to be a self-inflicted gunshot wound. She was still wearing her camouflage pants. The shotgun was found close to her body.

The FBI quickly posted an update to Twitter: “There is no longer a threat to the community.”

For Jefferson County School District officials, who planned to reopen schools Thursday, this was only partly true. One threat had ended.

But the anniversary — with its alumni reunions, service projects and other memorials — was still three days away.

Contrera, Shapira and Thebault reported from Washington. Lori Rozsa contributed to this report from Florida. Julie Tate, Mark Berman and Jennifer Jenkins contributed from Washington.