(CNN) - BREAKING NEWS UPDATE:
Sol Pais, 18, the Florida teenager who law enforcement authorities said was armed and was "infatuated" with Columbine is dead, the FBI said.
A sheriff in Colorado said Pais died of a self-inflicted gunshot wound.
The FBI had said that Pais flew to Colorado on Monday night and bought a pump-action shotgun and ammunition.
Authorities initially said she "made threats to commit an act of violence in the Denver metropolitan area."
Just days before the 20th anniversary of the Columbine High School massacre, authorities are scrambling to find an armed Florida teen they say is "infatuated" with Columbine and made threats as she traveled to Colorado.
The threats prompted Denver Public Schools and almost 20 other local school districts to close Wednesday.
A nationwide search is underway to find 18-year-old Sol Pais, who made "credible" -- but not specific -- threats after traveling from Miami to Denver on Monday night, said Dean Phillips, the special agent in charge of the Denver FBI office.
After Pais arrived in Colorado, she immediately went to a store and bought a pump-action shotgun and ammunition, the FBI said Tuesday night.
Pais is considered a threat to the community and schools, but there is no information on any specific threat to a particular place, Phillips said.
At least 19 school districts are closed .
Jefferson County Public Schools, which includes Columbine High School in Littleton, are among almost 20 school districts closed Wednesday as authorities search for Pais.
Other school districts shut down for the day include the Douglas County School District, Aurora Public Schools and Cherry Creek Schools.
In a letter to parents, Boulder Valley School District Superintendent Rob Anderson said the decision was unanimous among area school leaders.
"We couldn't take the risk of having one student or staff member injured -- so superintendents unanimously decided late last night to close school," Anderson wrote to parents Wednesday.
The wanted teen was last seen wearing camo
Pais is a student at Miami Beach Senior High School, said Daisy Gonzalez-Diego, spokeswoman for Miami-Dade County Public Schools.
"We are fully assisting the FBI in their investigation," the spokeswoman said. "The threats appear limited to schools in Colorado."
Gonzalez-Diego declined to give more details about Pais.
After the 18-year-old came to Colorado on Monday, she purchased a pump-action shotgun and ammunition and went to the foothills, where she was last seen, Phillips said.
"Her comments, her actions that we have heard about from others tend to cause us great concern that she may pose a threat to a school," he added.
Pais is about 5-foot-5 and was last seen wearing a black T-shirt, camouflage pants and black boots, authorities said.
Authorities said they are being especially cautious because Pais' statements don't express a specific plan and were mostly spoken.
"She did make statements that were threatening to schools and she did purchase a firearm ... and that's why she's a credible threat," said Patricia Billinger, a spokeswoman with the Colorado Department of Public Safety.
A man who said he is Pais' father told the Miami Herald he lost contact with his daughter Sunday.
"I think maybe she's got a mental problem," he told the Herald. "I think she's gonna be OK."
'This opens a wound'
On April 20, 1999, two students killed 12 of their schoolmates and a teacher in a mass shooting at the high school in the town of Littleton -- about 10 miles from Denver.
"I know that this opens a wound, especially on an anniversary week, for those families who were most deeply impacted by this," Jefferson County Sheriff Jeff Shrader said.
At this point, there might not be enough probable cause to arrest Pais. But federal and state officials are working to develop appropriate charges, Phillips said.
He said once they detain her, they will hold her for as long as they legally can.
Authorities said they have not identified any connections Pais has with Colorado. They are asking for anyone with information about Pais' whereabouts to call (303) 630-6227 or email firstname.lastname@example.org.
CNN's Deanna Hackney, Rosa Flores, Sheena Jones, Scott McLean and Darran Simon contributed to this report.
The Associated Press also contributed to this report.
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