JEFFERSON COUNTY, Colo. -- 23 Jefferson County Public Schools were placed on lockout Thursday morning after a reported threat at Columbine High School, the Jefferson County Sheriff's Office said.

The sheriff's office said that the threat came around 9:20 a.m. claiming that there were explosive devices inside the school. The caller claimed to be hiding outside with a gun.

Nothing has been found to validate the threat, according to the sheriff's office.

The 23 schools were in the Columbine, Chatfield and Dakota Ridge areas, JeffCo Public Schools said. Lockout means that students can only remain inside and that entry is restricted.

The sheriff's office said that parents were not allowed to pick up their children until they are released at 2:45 p.m.

Authorities said that all students and staff were safe.

"Unfortunately, we receive many threats like this to Columbine,” Mike Taplin with the sheriff's office said. "We respond to each threat appropriately, which is what we’re doing now."

The school was searched, the threat was considered fake, but the day was none the less nerve-racking for Nichole Tafoya, who waited outside the school for her 16-year-old daughter Brianna, who was waiting on the inside. "I was ready to lose it, yeah, just the fact that at that point the school had not sent anything out yet. It was just hearing from her that they were on lock out, watching the police tweets.”

Columbine Junior Leah Zundel was a little late to school Thursday morning. "There’s like two police cars there, with two cops that have guns, and you’re forced to turn left or right, I went left." She didn’t really mind the inconvenience, “That’s karma you sign up for it when you sign up for this school."

The all clear was given around 12:20 p.m.

The sheriff's office is continuing to investigate.