Mike Taplin, spokesman for the Jefferson County Sheriff's Office, told a press briefing that someone called in at 9:20 am (1620 GMT) with a threat of multiple explosive devices and a hiding gunman inside Columbine High School where 12 students and a teacher were killed in a mass shooting in 1999.

Video posted online showed that many police officers at the school and nearby area soon after the threat was made.

However, nothing dangerous has been found, Taplin said.

A total of 24 different schools in the county were placed on lockdown, which were lifted at 12:45 pm (1945 GMT) and students at Columbine High School were dismissed at the school's usual dismissal time.

The Colorado Office of Emergency Management confirmed that the school threat was linked to dozens of other similar threats nationwide which were dismissed by law enforcement agencies as incredible actions meant to cause disruption and compel recipients into sending money.

In these threats, the sender claimed to have planted 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 U.S. dollars in Bitcoin.

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