Updated 2:40 p.m. Monday: Oak Park students are being reunited with the parents after an anonymous threat prompted the lockdown and evacuation of the high school.
Between 1,300 and 1,400 students were in the building Monday, according to Jon Bazzano with the Clay County Sheriff's Office. They were bussed to Staley High School, where their parents could pick them up.
Aerial footage showed students exiting the school with their hands on their hands on their heads.
"During a threat like this, where we still don't know who our suspect is, it's standard protocol, and it's done out of an abundance of caution," Bazzano said at a news conference. "If a student, say, was in a small, confined room, and they were our suspect, we don't want them to come out with their hands in their pockets and have 200 or 300 students in front of them."
Bazzano said the sheriff's department actually conducted active shooter drills at Staley High School over spring break. Classes had only resumed Monday.
A search of the high school did not produce anything suspicious.
The search of Oak Park High School has been completed. Nothing suspicious was found. All roads have been reopened. @kcpolice
— Clay County Sheriff (@SheriffClayCo) March 27, 2017
Eight students were held for questioning.
Students will be allowed back into the building Tuesday morning. Bazzano said law enforcement officials were working on a plan to reunite students with their vehicles.
The original post continues below.
Students at Oak Park High School were evacuated Monday due to an anonymous threat.
The Kansas City, Missouri, Police Department's bomb squad searched the school but found no weapons. Students were led out with their hands on their heads as a precaution.
Several students were also held in the school's office for questioning, according to Jon Bazzano with the Clay County Sheriff's Office.
Students being evacuated now. Hands on heads, single file to buses outside . Oak park high school pic.twitter.com/fHTAclmLu6
— Christa Dubill (@christadubill) March 27, 2017
Bazzano says the threat was called in by someone who claimed to be a student. Authorities have not announced any suspects or charges filed.
"With threats like these, we're able to track down pretty quickly where the threat came from," Bazzano says. "In this case, it's taken us a while ... that's why we're going to take every precaution we can."
A statement by the North Kansas City School District said that the students would be taken to Staley High School so that law enforcement could continue to search the school.
Elle Moxley covers Missouri schools and politics for KCUR. You can reach her on Twitter @ellemoxley.
Cody Newill is the digital editor for KCUR 89.3. You can follow him on Twitter @CodyNewill.