Sirens were sounding, lights were flashing and first responders were swarming the area near Peconic Bay Medical Center Saturday morning as part of an emergency drill.

The hospital in Riverhead was staging a mass casualty drill, dubbed "Operation Black Swan 2018." The annual exercise brings hundreds of people together from multiple agencies -- including law enforcement, EMS, Homeland Security and public health groups -- to sharpen their skills in a simulated mass casualty event at the hospital.

In the simulated scenario, 25 people were injured by the "shooter" within three minutes, and multiple law enforcement and EMS agencies responded. Organizers say Riverhead police officers were on scene within the first 30 seconds.

The hospital remained open throughout the drill.

A portion of Peconic Avenue was closed for a time, but has since been reopened.