Concern mounted after investigators learned that upon her arrival at Denver's worldwide airport from Miami on Monday, Pais went to a store where she bought a pump-action shotgun and some ammunition.
The manhunt had led to the closing of Denver-area schools as a precaution.
The FBI had said Pais was "armed and dangerous" and believed to be somewhere in Colorado after making "credible" threats days before the 20th anniversary of the Columbine High massacre that ushered in the modern era of mass shootings at US schools.
Pais is a student at Miami Beach Senior High School, said Daisy Gonzalez-Diego, spokeswoman for Miami-Dade County Public Schools. He described her as "a very quiet, smart student who sat alone in class".
The FBI has said that Sol Pais, flew to Colorado on Monday night and bought a pump-action shotgun and ammunition.
Gonzalez-Diego declined to give more details about Pais. She was dead from a self-inflicted gunshot, Albers said.
All classes and extracurricular activities for about a half-million students were canceled as a precaution, though sheriff's spokesman Mike Taplin said the young woman's threats were general and not specific to any school.
He says he saw Pais frequently at school and that she mostly kept to herself.
Polis is referring to Sol Pais.
Officials said Thursday Pais' parents first reported her missing, prompting the investigation.
He says Colorado's "biggest priority is keeping our children safe".
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The newspaper cited a law enforcement source that it did not identify.
Authorities have said Pais traveled from Miami to Colorado Monday night.
State Rep. Patrick Neville, the Republican House minority leader, was a 15-year-old sophomore at Columbine High at the time of the shooting and now has three school-age daughters.
The doors were locked at Columbine and more than 20 other schools in the Denver area as the sheriff's office said it was investigating threats against schools related to an FBI investigation. Miami FBI agents on Tuesday night visited Pais' home.
Authorities said they believe she had been using a ride-sharing service in Colorado, and they don't believe she ever made it onto school property.
Authorities have identified the woman as 18-year-old Sol Pais.
Jefferson County Public Schools, which includes Columbine High School in Littleton, were among nearly 20 school districts closed Wednesday as authorities searched for Pais.
Because of the threats, Columbine and more than 20 other schools outside Denver locked their doors for almost three hours Tuesday afternoon, and some canceled evening activities or moved them inside.
"There is no longer a threat to the community", FBI Denver said in a tweet.
All schools in the Denver area were urged to tighten security, and some canceled evening activities or moved them inside. In the posts, the person asked for advice on how to buy a shotgun in Colorado.