The school closures came in the middle of the night amid credible threats against Denver area schools. 18-year-old Sol Pais was being sought by law enforcement before she was found dead with a self-inflicted gunshot wound.

DENVER — About a thousand schools canceled classes Wednesday in an unprecedented move while law enforcement searched for 18-year-old Sol Pais.

School cancellations began with Cherry Creek Schools and Douglas County Schools just before midnight - a few hours after law enforcement wrapped a press conference about the search for a woman accused of making threats toward schools in and around Denver. Within minutes of the first closures, several other districts posted their own notices including Denver, Aurora, Jefferson County, Douglas County, Englewood and more.

In notices sent to parents, Denver Public Schools said superintendents "collectively determined that the safest course of action will be to close schools." A tweet from the Colorado Department of Education tweeted summed up the reason: "Metro area school districts will be closed on Wednesday, April 17 due to ongoing safety concerns."

Pais' obsession with the Columbine High School shooting, the fact that she bought a gun after flying here from Miami during the week the shooting turns 20, and some concerning statements caused law enforcement in the state to go on a massive manhunt to find her.

She was found just before 11 a.m. with a self-inflicted gunshot wound and the shotgun she'd allegedly purchased from a store near Littleton.

Districts have announced that classes will resume Thursday, but students were already home when police told the public there was no longer a threat to the community. More than half a million students and their families were impacted by all of the closures, according to numbers from the CDE; 443,000 of them attend schools in the eight largest, metro area districts.

The education department first urged all schools in the Denver metro area to go into lockout status on Tuesday afternoon, after law enforcement first alerted schools to the threats. Jefferson County Public Schools, which includes Columbine High School, was notified around noon, said Superintendent Dr. Jason Glass.

"Got the threat a little before that," he said. "The message came out from the Commissioner of Education asking schools to put students in a controlled release. We had already made that decision. We were already operating in coordination with law enforcement. I can’t speak as to why they sent that out. In coordination with law enforcement, we made the right decision for our schools."

In a letter to parents, Jeffco said they worked closely with law enforcement to make decisions about dismissals. They explained the process like this:

"This situation stemmed from a threat that was identified by law enforcement, who notified Jeffco Public Schools Department of School Safety. In collaboration with Jeffco Sheriff’s Office, we placed selected schools on lockout as a precaution. Lockout means business as usual inside the buildings; entry and exit are restricted.

We know that situations like today are concerning. While not common, sometimes schools go on lockout during the day if there is a safety concern outside of the school building. Most of the time, these last a few minutes while the issue is resolved. When a lockout lasts only a few minutes, we send a message to families at the conclusion of the incident. Other times, like today, we keep schools in lockout status for an extended period. With an extended lockout, we must confirm the action plan with the Jeffco Sheriff’s Office and follow their direction before notifying families. We posted immediate updates on social media and then followed with a more detailed message as more information was available."

Glass said threats have ramped up a bit lately, as we approach the 20th anniversary of the shooting at Columbine. He said they're disruptive and distracting. Still, he felt the district handled the events as well as they could have, partially and unfortunately because of previous experience.

"Jeffco has the gold standard model in security and response protocols. That comes with the history of Columbine, and our school district experiencing two other school shootings. We have a robust system of school safety and security. We are hypervigilant. We take all these concerns very seriously. We also don’t want to overreact to them. Keep the focus on the learning experience while we also pay attention to safety," he said.

Frank DeAngelis was the principal of Columbine at the time of the shooting. He was at the school when they were alerted to the threats. DeAngelis said in a Wednesday press conference that the students and staff impressed him in the moments after they found out what was happening.

"The one thing that is so different from this time as opposed to 20 years ago is all the things that we have in place. Prior to that time, the only drills we did were fire drills. And yesterday, as soon as there was an alert to go into lockdown, Columbine High School acted so professionally. And the reason they did this is because of the training being done by the Jeffco Sheriff’s Dept. and John’s office. And so it was like clockwork. And I was there through the entire time and the kids knew exactly what to do," DeAngelis said.

Colleges including the University of Colorado campus in Boulder, Metropolitan State University of Denver and the Auraria Campus remained opened Wednesday, saying there was no indication that a threat had been made against higher education. Officials at these schools said they either increased the police presence or maintained close contact with law enforcement, as a precaution.