First, just three officers respond. Then, another cruiser arrives. Then, an army of police, weighed by their rifles and swat gear, storm a local high school under attack.
"These things are evolving and dynamic," says Major Mark Person of the Prince George's County Police Department. "In most active shooter situations, there are less than two or three officers that have to go in that building initially."
More than two hundred police, Fire and EMS officers from Prince George's County, Montgomery County and other agencies are taking part in an active shooter drill. Police are combing the halls for four armed suspects, as would-be-students hide, then rush for the doors.
"This is pretty stressful. This is pretty intense," Person says. "Training builds confidence and that's what we're trying to do here. We want our officers to feel confident that they can go into a situation like this and be able to survive and also save lives."
The confidence to stay calm, think, and communicate can be critical in an emergency scene. For the first time, police, the school and the fire department are sharing a radio frequency - a way to pull all the agencies together into one strategy.
It's something learned from the tragedy in Aurora, Co., when an active shooter terrorized the century 16 movie theater during a screening of the latest batman film. There, multiple police agencies were able to talk to each other over radio without switching frequencies.
The practice commonly referred to as "mutual aid" channels was largely a response to communication challenges that occurred during the 1999 shooting at Columbine High School.
"Unfortunately in the world we live in today, we have to be ready for a situation like this," says Julie Parker, spokeswoman for the Prince George's County Police Department. "This is exactly why we're doing this drill. We hope to never need this training, but we have to prepare."
On the eve of the one-year anniversary of the Aurora shooting, fake blood and pretend casualties are a chilling reminder that this drill could be reality.
"I think it went very well," Person says. "I can't wait to critique this incident because I think we'll have a lot of positives that come out of it."