Denver closed all of the city’s public libraries Wednesday and an Adams County high school canceled classes after receiving threats amid a surge of hoax calls and threats of violence both in Colorado and across the nation.

The Denver Public Library system received an unspecified “digital threat” on Tuesday night, library officials said in a statement, and Adams City High School officials became aware of a threat of violence circulated on social media Tuesday night, according to the school district.

Both organizations opted to cancel the next day’s operations, which included closing 25 branch libraries across Denver and halting bookmobile operations.

The disruptions are the latest in a rash of similar threats: several Colorado schools received hoax calls about supposed active shooters on Monday, including at Denver’s East High School, and at schools in Colorado Springs and Alamosa.

Similar bogus calls about active shooters at schools have been seen in at least 10 other states during the last couple of weeks, according to the National Association of School Resource Officers.

And the same goes for libraries. In Texas, public libraries in Forth Worth were evacuated Tuesday over a bomb threat, according to the Fort Worth Star-Telegram.

Preliminary information indicates the threat against the Denver Public Library system came from outside of Colorado, Denver police spokesman Sean Towle said Wednesday.

“Similar threats have been sent to libraries in other states,” he said, adding that police will conduct extra patrols around library locations as a precaution.

Denver Public Library officials said they plan to reopen all locations Thursday except for branches currently closed for renovation or maintenance.

Vikki Migoya, a spokeswoman for the FBI in Denver, said Monday that the agency was aware of “numerous swatting incidents” in which someone falsely reported an active shooter at a school and said the agency was working with local law enforcement to investigate some of the recent threats. She declined to comment further Wednesday.

Adams City High School also was placed on lockdown Monday afternoon and a student was taken into custody, after which the school’s principal asked parents to “take this opportunity to speak with your student about the dangers of bringing a weapon of any kind to school, or even threatening to use a weapon.”

“Threats have to be taken seriously”

The unfounded threats have put many parents and educators on edge, said Randy Barber, a spokesman for the Boulder Valley School District. Last week, the Boulder County Sheriff’s Office served a search warrant on a man in Superior and seized a handgun, journals and “digital evidence” as part of an investigation into online threats of a school shooting and “racially biased material.”

No arrests have been made in that case, and while the Boulder Valley School District has not received a hoax call about an active shooter this week, the school district still alerted parents and principals to the possibility of such a hoax, Barber said.

“It felt like the next day we might have one,” Barber said. “There were enough of these happening nationally that it felt like we could be next.”

The preemptive warning was intended to help everyone react appropriately if such a report were to come in, he said, adding that the district routinely evaluates threats and reports of threats for credibility.

“For swatting, you are letting your principals know so they are aware that it could happen, and so that if it happens they can be in the mindset of — you still have to react the same way because you just don’t know — but you can have it in your mind, ‘Can we be clear it’s not real and how quickly can we do that?’”

An influx of vague or hoax threats puts law enforcement and school officials in a tough spot, said Sarah Goodrum, senior research associate at the University of Colorado Boulder’s Center for the Study and Prevention of Violence.

“What’s challenging in these situations is that not all threats are serious, but all threats need to be taken seriously,” she said, adding that authorities should consider specific factors like whether the person making the threat has access to guns or harbors a grievance with the targeted school — and that’s tough to quickly figure out if the threat-maker is anonymous.

She added that the person or people making the threats may be fueled by each new threat.

“There is probably some sort of contagion factor,” she said. “…Certainly there’s evidence to suggest that people who have perpetrated a school attack had a fascination with other school attackers.”

“Not something we’ve seen”

Mo Canady, executive director of the National Association of School Resource Officers, said that while publicizing hoax calls might encourage additional hoaxes, law enforcement must initially treat each threat as though it is real.

“Until we know it is a false report, we have to treat it as if it’s real,” he said.

But that sort of high-intensity response is dangerous to everyone involved and harmful to students, he added. Officers or parents rushing to the scene can make mistakes or get hurt.

“It’s very traumatizing to students,” Canady said. “That’s a very traumatic situation where you think something is really going on and then all the sudden you’re in a lockdown.”

He did not know what might be driving current the rash of hoax calls about active shooters, but said he began to notice the trend about 10 days ago, first in Texas before it moved across other states. While bogus bomb threats have been around for decades, false calls about active shooters are new, he said.

“It’s not something we’ve seen, in large part, before now,” he said.