The FBI said Sol Pais, 18, is “considered to be extremely dangerous” and “made threats to commit an act of violence in the Denver metropolitan area” just days before the 20th anniversary of the attack that killed 13 people.

All schools in the Denver area were urged to tighten security because the threat was deemed “credible and general,” said Patricia Billinger, a spokeswoman for the Colorado Department of Public Safety. Columbine and more than 20 other schools outside Denver lock their doors for nearly three hours Tuesday afternoon before Wednesday’s complete closures were announced.

The Jefferson County Sheriff’s Office and the FBI say Pais traveled to Colorado from Miami on Monday night and bought a pump-action shotgun and ammunition.

Denver Public Schools said that all facilities and programs will be closed Wednesday, and there will be no afternoon activities or athletic competitions. The district said the decision to close campuses was in collaboration with other Denver metro-area school districts due to the ongoing safety concern.

On Tuesday, some schools released their students after additional security was called in and canceled evening activities or moved them inside.

“We always have heightened awareness close to high-profile anniversaries like this,” Billinger said.

Authorities said Pais was last seen near Columbine — in the Jefferson County foothills outside Denver — wearing a black T-shirt, camouflage pants and black boots. They appealed for anyone seeing her to call an FBI tip line at 303 630-6227, and said she is too dangerous to be approached by civilians. The alert also said police who come into contact with her should detain her and evaluate her mental health.

“This has become a massive manhunt … and every law enforcement agency is participating and helping in this effort,” Dean Phillips, special agent in charge of the FBI in Denver, said late Tuesday night.

The FBI’s Rocky Mountain Safe Streets Task Force issued a notice Tuesday describing Pais as “infatuated with (the) Columbine school shooting.”

Sheriff’s spokesman Mike Taplin said the threats she made were general and not specific to any school.

The Denver Post reported that a call to a phone number listed for Pais’ parents in Surfside, Florida, was interrupted by a man who identified himself as an FBI agent and said he was interviewing them.

Surfside Police Sgt. Marian Cruz confirmed that her parents last saw her on Sunday and reported her missing on Monday. The Miami Herald and WTVJ are reporting that neighbors say the teen is a senior at Miami Beach High School.

The Associated Press left messages at two numbers listed for Pais’ relatives in Florida, while another number was disconnected.

Two teenage gunmen attacked Columbine on April 20, 1999, killing 12 classmates and a teacher.

LINCOLN, Neb. (AP) — A man wanted in the 2016 shooting death of a man in Mississippi has been arrested more than 700 miles away in Lincoln, Nebraska.

The U.S. Marshals Service says in a news release that its Metro Fugitive Task Force arrested 25-year-old Stephon Clay, of Clarksdale, Mississippi, on Monday night in Lincoln. Members of the Lincoln and Lancaster County Narcotics Task Force and the Lincoln Police Department helped in the arrest.

Clay is suspected in the March 19, 2016, shooting death of 22-year-old Steven Harris in Clarksdale.

A news release from police says the helicopter, Able-1, had just left its base at Omaha North Airport when the emergency happened.

Police say the pilots took immediate action and landed in an open field just northwest of the airport. No one was injured.

The department still has two other aircraft. Police say the helicopter that made the emergency landing was a military-acquired Bell OH58.

The Federal Aviation Administration is investigating the incident.

The Sioux City Journal reports Derek Olson pleaded no contest Monday to second-degree murder and second-degree arson in the March 10, 2017, death of 64-year-old Ernest Warnock. Authorities found Warnock’s body in the burned rubble of his home in rural Rosalie.

Prosecutors will recommend Olson be sentenced to 40 to 60 years in prison for murder and four years for arson. He will be sentenced July 18.

A mistrial was declared in an earlier trial of Olson after an investigator mentioned a lie detector test taken by a witness.

Olson’s father, Jody Olson, pleaded guilty earlier to second-degree murder.

Two others also have been convicted of charges related to the killing.

Experts say the combination of a structure that’s more than 850 years old, built with heavy timber construction and soaring open spaces, and lacking sophisticated fire-protection systems left firefighters with devastatingly few options Monday once the flames got out of control.

“Very often when you’re confronted with something like this, there’s not much you can do,” said Glenn Corbett, a professor of fire science at John Jay College.

Fire hoses looked overmatched — more like gardening equipment than firefighting apparatus — as flames raged across the cathedral’s wooden roof and burned bright orange for hours. The fire toppled a 300-foot (91-meter) spire and launched baseball-sized embers into the air.

While the cause remains under investigation, authorities said that the cathedral’s structure — including its landmark rectangular towers — has been saved.

Some of the factors that made Notre Dame a must-see for visitors to Paris — its age, sweeping size and French Gothic design featuring masonry walls and tree trunk-sized wooden beams — also made it a tinderbox and a difficult place to fight a fire, said U.S. Fire Administrator G. Keith Bryant.

With a building like that, it’s nearly impossible for firefighters to attack a fire from within. Instead, they have to be more defensive “and try to control the fire from the exterior,” said Bryant, a former fire chief in Oklahoma and past president of the International Association of Fire Chiefs.

“When a fire gets that well-involved it’s very difficult to put enough water on it to cool it to bring it under control,” Bryant said.

And while there’s a lot of water right next door at the Seine River, getting it to the right place is the problem, he said: “There are just not enough resources in terms of fire apparatus, hoses to get that much water on a fire that’s that large.”

Because of narrower streets, which make it difficult to maneuver large ladder fire trucks, European fire departments don’t tend to have as large of ladders as they do in the United States, Bryant said.

And what about President Donald Trump’s armchair-firefighter suggestion that tanker jets be used to dump water from above on Notre Dame?

French authorities tweeted that doing so would’ve done more harm than good. The crush of water on the fire-ravaged landmark could’ve caused the entire structure to collapse, according to the tweet.

Other landmark houses of worship have taken steps in recent years to reduce the risk of a fire.

St. Patrick’s Cathedral in New York City, built in 1878, installed a sprinkler-like system during recent renovations and coated its wooden roof with fire retardant. The cathedral also goes through at least four fire inspections a year.

Washington National Cathedral, built in 1912 with steel, brick and limestone construction that put it at less risk of a fast-moving fire, is installing sprinklers as part of a renovation spurred by damage from a 2011 earthquake.

That cathedral faces fire inspections every two years, but D.C. firefighters stop by more often to learn about the church’s unique architecture and lingo — so they’ll know where to go if there’s a fire in the nave, or main area of the church — for instance.

“It’s really important for us to make sure that those local firefighters are aware of our building and our kooky medieval names that we use for all the different spaces and that they know where to go,” said Jim Shepherd, the cathedral’s director of preservation and facilities.

Cardinal Timothy Dolan, the New York Archbishop who often visited the Notre Dame Cathedral while studying in Europe, saw significance in the fact that the fire broke out at the beginning of Holy Week, when Christians there and around the world prepare to celebrate Easter and the resurrection of Jesus Christ.

“Just as the cross didn’t have the last word, neither — for people of faith in France — will this fire have the last word,” Dolan said.

Police say investigators on Monday arrested 19-year-old Marvin Rivas-Villanueva, who will be charged with motor vehicle homicide in connection with the March 29 crash. A passenger in Rivas-Villanueva’s car, 23-year-old Jared Williams of Lincoln, was injured in the crash and was pronounced dead at a hospital.

Rivas-Villanueva and two others were injured.

The crash occurred when Rivas-Villanueva lost control and spun into oncoming traffic. Police contend he was driving at least 70 mph on a wet roadway.

The flooded Loup River battered dams, bridges and roads last month, splitting the Twin River Public Schools district in two.

Many students and teachers in the district that serves Genoa, Silver Creek and Monroe are isolated from their schools as two main river crossings have become impassable indefinitely. The typical 10-minute drive between Silver Creek and Genoa now calls for a maze of detours and muddy back roads that can take about 75 minutes.

The historic floods have caused an estimated $1.4 billion in damage across Nebraska, drawing federal disaster assistance.

Superintendent John Weidner Sr. said the district couldn’t wait months for the floodwaters to recede so crews could repair the damage. He said administrators quickly hatched a plan to direct students and teachers stranded north of the Loup River to a Genoa school, while those south of the floods would report to an underused elementary school in Silver Creek.

Teachers were trained to deliver lessons though video conferencing, while some swapped roles with their counterparts living in the towns that they formerly commuted to. Teachers email tests and communicate with students by Facebook and text messaging.

“We had to do something to get the school up and running again,” Weidner said. “We thought we could pull this off.”

A main highway splitting the towns isn’t expected to reopen until next fall, so the district plans to salvage the remaining weeks of the 2018-2019 school year through the virtual classes.

Kirk Hebda, a 13-year-old student who is attending the alternative school in Silver Creek, said the distance learning can make certain classroom interactions difficult.

Kirk said “in some classes it’s harder than others to learn what the teacher’s teaching.”

“Like in math, you can’t interrupt his lesson to ask him questions,” he said. “And in art right now, we’re working with paint. You have to mix paint. So you don’t know if you have the right color or not that the teacher wants.”