Colorado Gov. John Hickenlooper, left, stands with state Senate President-elect Morgan Carroll, D-Aurora, left, and House Speaker Rep. Mark Ferrandino, D-Denver, during a news conference at the Capitol in Denver, Tuesday Jan. 7, 2014, which they discussed the bipartisan Safe2Tell Act, a measure to fund and run a successful school danger hotline.

LITTLETON, Colo. - Columbine High School, where a school shooting left 13 people dead in 1999, went on high security alert after receiving a series of threatening phone calls, law enforcement said Thursday.

The school received the first phone call at 10 a.m. Thursday and others came "over the next 2-3 minutes," said Jacki Kelley, a spokeswoman for the Jefferson County Sheriff's Office.

The school was placed on lockout, which prevents people from entering or leaving but allows them to freely circulate inside. The alert also applied to a half-dozen other schools in the area and was issued shortly before noon. The lockout was lifted shortly after 2 p.m.

Kelley said the sheriff's office sent "additional resources" to protect the school and was investigating the threats. The department added in a tweet that the threat "does not appear to be local or credible."

In 1999, two students opened fire at Columbine, killing 12 classmates and a teacher and wounding 26 others before killing themselves in the school's library.