Phillips said the threats she made were general, not specific to any school.
Ten minutes later, the district followed with more information: "To clarify, mountain schools include Evergreen High, Evergreen Middle, Conifer High, West Jeff Middle, West Jeff Elementary, Bergens, Wilmot Elementary, Marshdale Elementary, Elk Creek Elementary, Ralston Elementary, Rocky Mountain Academy, Parmalee Elementary".
The Jefferson County Sherriff's Office today confirmed an 18-year-old woman named Sol Pais travelled to Colorado and made threats that sparked the lockdown. She was last seen wearing a black t-shirt, camouflage trousers and black boots and was last seen in the Jefferson County foothills.
The 20th anniversary of the attack is on 20 April. The FBI and other authorities are looking for an 18-year-old woman who is believed to be connected to the lockouts and who is considered armed and unsafe, according to the FBI. Students at high schools across the country are expected to walk out of classes Friday, the 19th anniversary of the Columbine shooting, in their latest push for gun control.
The safety unit run by Jefferson County Public Schools is considered one of the most sophisticated school security systems in the country.
Jefferson County Sheriff's Department tweeted about the lockdown mid-afternoon Tuesday. "I think she's going to be OK", the man said.
Please call the Federal Bureau of Investigation tipline at (303) 630-6227 where your call will be answered immediately if you have seen this individual or have information on her whereabouts.
The FBI is leading the investigation, said Mike Taplin, a spokesman for the sheriff's department.
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Officers were "investigating what appears to be a credible threat possibly involving the schools", the sheriff's department said on Twitter, adding that students were safe and additional deputies were dispatched to the schools.
That system was tested in December, when Columbine High School went into lockout for hours on a Thursday.
Students left classes on time, but after-school activities were canceled at Columbine in Littleton, Colorado. They described her as "armed" and "extremely dangerous" and said she was last seen in the county's foothills, clad in camouflage trousers, black boots and a black T-shirt.
In the years since the massacre, the shooting has become a grim obsession for people known as "Columbiners".
Authorities said they have not identified any connections she has with Colorado and most of her threats were spoken.
In a tweet, the Jefferson County Sheriff's Office said, "We are now investigating what appears to be a credible threat possibly involving the schools".
Schools districts across the Denver metro area were placed on lockout after the threats; the Post reported that over 20 schools were affected.