JEFFERSON COUNTY, Colo. (CBS4) – Nothing dangerous has been found after a threat of explosives was called in about Columbine High School on Thursday morning. A total of 24 different Jefferson County schools were placed on lockdown. Those lockdowns were lifted a few hours later and at 12:45 p.m., it was determined the threat at Columbine was not substantiated.

The high school’s perimeter was secured and students and staff were declared to be safe, but officials asked people to avoid the area around the school in the hours afterwards.

> The FBI is reporting that 25+ states have received these threats, which have all been unsubstantiated. No bombs have detonated.

> The FBI is aware of the recent bomb threats made in cities around the country. — CO – Emergency Mgmt (@COEmergency) December 13, 2018

The school threat was linked to dozens of other similar threats nationwide, according to the Colorado Office of Emergency Management.

A wave of bomb threats emailed Thursday to hundreds of schools, businesses and government buildings across the U.S. triggered searches, evacuations and fear. Law enforcement agencies across the country dismissed the threats, saying they were meant to cause disruption and compel recipients into sending money and were not considered credible.

Some of the emails had the subject line: “Think Twice.” They were sent from a spoofed email address. The sender claimed to have had an associate plant a small bomb in the recipient’s building and that the only way to stop him from setting it off was by making an online payment of $20,000 in Bitcoin.

Mike Taplin, spokesman for the Jefferson County Sheriff’s Office, said someone called in the threat at 9:20 a.m.

“We received a phone call with a threat of multiple explosive devices inside the school here. The person claimed to be hiding outside with a gun. The perimeter was searched and secured. No one was found,” Taplin said.

“So far we’ve not found anything to validate the threats that have been made.”

All of the following schools were locked down:

Blue Heron Elementary School

Bradford K8 North

Bradford K8 South

Chatfield High School

Colorow Elementary School

Columbine Hills Elementary School

Columbine High School

Coronado Elementary School

Dakota Ridge High School

Deer Creek Middle School

Dutch Creek Elementary School

Falcon Bluffs Middle School

Governor’s Ranch Elementary School

Ken Caryl Middle School

Leawood Elementary School

Mount Carbon Elementary School

Mortensen Elementary School

Normandy Elementary School

Powderhorn Elementary School

Shaffer Elementary School

Summit Ridge Middle School

Stony Creek Elementary School

Ute Meadows Elementary School

Westridge Elementary School

Students at Columbine High School were dismissed at 2:45 p.m. the school’s usual dismissal time.

Soon after the threat was made, video from Copter4 showed a large police presence at Columbine and at nearby Clement Park, where there is a memorial to the victims of the mass shooting at Columbine High School in 1999. Officers could be seen near the memorial with rifles drawn. Numerous police cars could also be seen around the school.

Police also blocked off an intersection near the front of the school.

.@jeffcosheriffco bomb squad truck just arrived on scene at Foothills Park due to area threat. A reported suspicious person is near #Columbine High School. Schools are on lockout. @CBSDenver pic.twitter.com/9Pq8jblxjz — Melissa Garcia (@MelissaGarciaTV) December 13, 2018

The police response was one of the biggest seen at Columbine due to a school threat since 12 students and a teacher were killed inside the school on April 20, 1999.

“Unfortunately we receive many threats like this to Columbine. We respond to each threat appropriately, which is what we’re doing now,” Taplin said.

By noon, the number of police officers at the scene of the school had noticeably diminished. The school threat was deemed unsubstantiated at 12:45 p.m.

“At one point, we did have a security guard come in and tell us we needed to get against the wall,” said Thomas Parker, a sophomore at Columbine.

He was relieved when he learned that the threat of a gunman with bombs appeared to be unfounded.

Parker was also grateful for the school’s security measures.

“I know that Jeffco does a lot here, especially with the implementations of security guards and always police officers here. So I felt better,” he told CBS4’s Melissa Garcia.

Family members picking up their students said that seeing law enforcement agencies’ quick team response helped them to get through the day’s tense moments.

“Law enforcement was here checking the area, making sure it was all safe,” said Edward Valdez, as he was waiting to pick up his granddaughter from school.

He hoped that officers would catch whoever made the threat.

“If they find him, I think something should happen,” Valdez said.

Investigators were looking into a possible connection between the Columbine threat and other threats that happened the same day across the country.

“As always, we encourage the public to remain vigilant and to promptly report suspicious activities which could represent a threat to public safety,” the FBI said in a statement.

With contributions from Melissa Garcia. She has been reporting for CBS4 News since March 2014. Find her bio here, follow her on Twitter @MelissaGarciaTV, or send your story idea to mkgarcia@cbs.com.