A US teenager whose obsession with the Columbine High School massacre set off school closures and a manhunt in the Denver area has been found dead from an apparently self-inflicted gunshot wound, officials said.

Media reports said 18-year-old Sol Pais's body was discovered near the base of Mount Evans, which dominates the Denver skyline.

Jeff Shrader, the Jefferson County sheriff, told a news conference that Ms Pais was found "deceased from an apparent self-inflicted gunshot wound" by the FBI, which separately confirmed her death.

"They are currently processing the scene where her body was found to determine all of the factors that would lead to those conclusions," he said.

"I don't believe that they were in active pursuit at the time that she was found dead," he added.

Authorities had mounted a massive search for Ms Pais after she travelled from Florida to Colorado, bought a pump-action shotgun and ammunition and was last seen in the foothills around Denver.

Ms Pais was not even born when two Columbine students launched their attack with an assortment of guns and bags-full of pipe bombs, killing 12 fellow students and a teacher before taking their own lives.

Security fears increased because Saturday is the 20th anniversary of what at the time was the deadliest school shooting in US history, setting off an era of similar massacres at schools and public venues.

Colorado authorities had ordered around half a million students to stay at home "due to ongoing safety concerns."

Dean Phillips, the special agent in charge of the FBI's Denver office, said Ms Pais had made comments in the past that expressed "an infatuation" with the Columbine massacre and its perpetrators.

Concern mounted after investigators learned that upon her arrival at Denver's international airport from Miami on Monday, Ms Pais went to a gunshop where she bought a pump-action shotgun and some ammunition.

She was last seen on Monday night in the foothill areas around Denver, he said.

"We have been trying to find her ever since," Mr Phillips told a news conference late Tuesday in Denver.

He said Ms Pais was considered a "credible threat" but noted the FBI had no information of a specific threat either to Columbine or any other school.

"She has had an infatuation with Columbine, with the perpetrators of Columbine, she has made comments to that effect, but she hasn't identified a specific threat to a specific school," Mr Phillips said.

The Miami Herald reported that Ms Pais lived in Surfside, Florida and that her parents had reported her missing Monday night.

Her father told the newspaper he thought "maybe she's got a mental problem," but added: "I think she's gonna be OK."

Every law enforcement agency in the state took part in the hunt for the teen.

Columbine has been followed by even deadlier school shootings - at Virginia Tech in Blacksburg, Virginia in April 2007; Sandy Hook Elementary School in Newton, Connecticut in December 2012; and Marjory Stoneman Douglas High School in Parkland, Florida in February 2018.