Police investigate threats to Colorado's Columbine High School DENVER (Reuters) - Columbine High School in Colorado, scene of a 1999 shooting rampage in which a teacher and 12 students were killed, was placed on "lockout" for several hours on Thursday as police investigated threats, a school district spokeswoman said. The nature of the threats was not immediately made public. A spokesman for the Jefferson County Sheriff's Office said three telephone threats had been made to the school around 10 a.m. local time. The lockout was lifted several hours later, and the sheriff's office said in a tweet that the threats were determined not to be local or credible. The lockout prevented anyone from entering or leaving the school, although classes remained in session, Jefferson County Sheriff's spokesman Mark Techmeyer said, adding that there was an increased police presence at the school. Techmeyer said threats made to Columbine High School were not uncommon. Six other Jefferson County public schools were also placed on lockout following the threats, according to the Jefferson County Public School District. "All students and staff are safe," the district said on its Twitter feed. In April 1999. Two heavily armed Columbine High School students, Eric Harris and Dylan Klebold, opened fire at the school, killing a teacher and 12 classmates before committing suicide. Reporting by Keith Coffman; Writing by Dan Whitcomb; Editing by Cynthia Johnston, David Gregorio and Steve Orlofsky