DENVER (KDVR) – What we saw at Northfield High School in Denver on Thursday was a lockdown, the most serious response to a threat at school.

The Problem Solvers learned of multiple strategies Denver Public Schools deploys in an emergency.

The first is relatively new, according to the district. It’s called ‘secure perimeter.’

“There may be a threat, it may be direct to the school, it may not be,” Scott Pribble, spokesperson for DPS, said.

This is the most common when there’s a threat to a school or more often a possible violent crime near a school.

“When that happens we make sure that all of our exterior doors are closed and locked, we make sure that if there’s somebody outside on a playground or outside of the building that they come inside the building,” Pribble said.

In this kind of event, Pribble said, class is still in session, educating proceeds albeit behind a locked door and on a secure campus.

The next and highest level of securing a campus in an emergency is lockdown, where a perceived threat might be inside a school.

“All the classroom doors should be locked, lights off, students and staff huddled in a place where they’re not easily visible from the door if somebody happens to walk by,” Pribble said.

These plans are fluid and subject to change, Pribble said, as the district’s security team assesses new strategies using other incidents around the county as reference points.

“[We do this] to make sure that our plan is up to date, to make sure the things that we’re doing are correct, and that it’s best for our students and staff,” Pribble explained.

So how exactly was the paintball gun able to make it onto the campus?

“I don’t know that we have an answer to that at this point, we’ll continue to take a look at what do we need to do to ensure that our students and staff are safe,” Pribble said.

There is another important point for parents to understand, in an emergency, DPS told FOX31, and that’s that communication takes secondary priority.

It could feel like an inordinately long amount of time before any information is shared, but the district said this is by design, as securing students from any threat is top priority in an emergency.