Nassau police started conducting several days of active shooter exercises Thursday on an empty floor of North Shore University Hospital. 

Each scenario lasted just a few minutes, but officers' decisions had to be made in a split second. Acting Nassau Police Commissioner Patrick Ryder says the training is crucial.

"When you move down that hallway, the screaming and yelling, it gets your heart rate up, so you're trying to get the cop to control his emotions," says Ryder. 

News 12's Eileen Lehpamer, as well as staff at the hospital, also participated in the drills. 

Deputy Executive Director for Hospital Operations Jon Sendach says the lessons learned are also invaluable for hospital employees. 

Nassau officers had previously trained for an active shooter situation at Nassau Community College.