Columbine High School and about 20 other Denver schools were placed on lockout Tuesday afternoon after an 18-year-old girl made a “potential credible threat,” authorities said. The lockout, which prevented people from entering and exiting the schools, began around 2 p.m. The FBI and local authorities were investigating the threats, a Jefferson County sheriff’s spokesman said. “All students and staff are safe. Students will be released from schools normally; buses will be running on normal schedule,” the Jeffco school district tweeted on Tuesday. “We will have extra safety and security staff on site at all schools affected.” The school district added: “All after-school activities, sports, and practices will occur as scheduled EXCEPT for Columbine High School where all such activities are canceled.”

Later Tuesday, Jefferson County Sheriff’s Office said 18-year-old Sol Pais was behind the threats. In tweets, police called Pais “armed” and “extremely dangerous,” and said she “traveled to Colorado & made threats” on Monday night. She was last seen in the “foothills of Jefferson County” wearing a “black t-shirt, camouflage pants, and black boots,” police said. The threats come just days before the 20th anniversary of the mass shooting at Columbine High School that killed 12 students and a teacher.