DENVER (KDVR) — Many people showing up at Denver public libraries across the city on Wednesday were surprised to find the doors locked. Turns out, an overnight threat led to the closure.

Libraries are among the latest to deal with threats forcing them to close their doors.

“I’m very shocked. I was surprised, “ library patron Angelo said.

Twenty-five of the city’s public libraries were closed due to an unspecified threat received overnight.” A spokesperson said it was a “digital threat.”

“I said to myself, rats. It’s ridiculous. Unnecessary,” library patron Peter Mayerson said.

The shutdown comes after an unfounded swatting threat was made at East High School on Monday. Other schools have been shut down this week in Colorado because of what is believed to be swatting.

What makes swatting hard to fight?

Kevin Kolbye is a former FBI agent who investigated swatting incidents across the country.

“The issue is there is no uniform crime reporting by police departments or even federal agencies with swatting as a particular crime, like a robbery or theft or something like this,” Kolbye said.

He said not categorizing swatting as swatting makes the threats hard to count and fight.

“What happens is a lot of times the swatting incidents are just reported to a police department or a sheriff and they look at it and say, ‘fictitious report,’” Kolbye said.

Kolbye said stronger laws are needed to make sure people serve jail time when they are caught swatting. The goal would be to slow down threats that cause problems and put people in danger.

The library said they are hoping to reopen Thursday. Their main goal is to keep library patrons and staff safe.

Denver police say extra patrols will be checking on area libraries because of the threat. FOX31 is also being told the threat appears to have come from out of state.

Threats were also made to libraries in other states.