Locked and loaded with both Nerf guns and balled-up socks, students from around the state gathered at the University of Colorado's engineering building Saturday night for a game of hunting zombie specimen that escaped from the lab around 11 p.m.

The stairwells are the worst -- dark and twisted with plenty of nooks and crannies for the zombies to hide. Six humans, marked by bandanas around their arms, surround the stairwell door, before one of them kicks it in, immediately ducking to avoid being attacked by any lurking zombies, recognized by bandanas on their heads.

"All clear!" one of them yells as they jog down the stairs to the first floor without so much as lowering their weapons. Taking a deep breath, the leader motions to the others to quietly follow him into the long dark hallway, realizing that the slightest sound could give them away.

Human teammates -- led by CU juniors Arthur Zimmerman and Scott Mclean -- made up of students from CU-Boulder, CU-Denver and Colorado State University set out on a zombie chase Saturday night during a "top secret" session of Humans versus Zombies -- the popular college game of "tag."

This was the second zombie invasion in a month at the University of Colorado, following a weeklong game earlier this month that hosted more than 200 players.

Colorado State University junior Bonnie Goss covers her squad in a game of Humans vs. Zombies in the CU Engineering Center on Sunday morning.
Colorado State University junior Bonnie Goss covers her squad in a game of Humans vs. Zombies in the CU Engineering Center on Sunday morning. ( Nicholas Duckworth )

But Saturday night's event was no ordinary game. The university allowed players to use Nerf guns for the first time since December, when officials enforced a campus policy and banned the use of simulated weapons during play.

The university Police Department issued the students a temporary weapons waiver for the weekend session because it was played in a "controlled and secure" building, where bystanders were clearly informed about the game and the purpose of the weapons, according to police spokeswoman Molly Bosley.

"I wouldn't go as far as calling it a reward but their ability to cooperate with us and coming into the meetings organized and willing to work with our concerns, made us more willing to work with them too," Bosley said.

University of Colorado freshman Josh Bromberg, also known as a  zombie,  searches for brains in a game of Humans vs. Zombies in the CU Engineering Center
University of Colorado freshman Josh Bromberg, also known as a zombie, searches for brains in a game of Humans vs. Zombies in the CU Engineering Center in the early morning hours on Sunday. ( Nicholas Duckworth )

The game was kept "top-secret" according to HvZ organizers and was only played by approved students who received a personal invitation to join the fun.

"By keeping the participation limited, the game will be easier to control and we want to keep it as organized as possible so that the university will continue to cooperate and let us play," HvZ organizer Scott Serafin said before the game, which the zombies eventually won.

Players said the indoor game was much more intense than the weeklong university-regulated game played earlier in the month, because the confined location allowed for "suspense around every corner."

"Being able to use the guns and play the game consistently for five hours is really intense," Zimmerman said. "What a way to start spring break."

The late-night mission was confined to the first and second floors of the engineering building, with permission from the college to lock down the space as long as a security guard was on duty and bystanders were properly informed of the game.

Though an exception was made for the weekend mission, both university officials and game organizers said they were not encouraged to think the weapons ban or other regulations, such as area and time restrictions, would be removed for future extended games.