It's not uncommon for counter-terrorism forces to use real facilities in their drills and on Friday night, the US Marine Corps used Barclays Center along with the Fire Department of New York and the New York Police Department.
According to the Brooklyn Paper, the scenario went like this
The attack scenario, according to Hall, is a plot with every element planned to the T: a fake president and first lady will be in attendance at a packed fake basketball game [between the Nets and Celtics] that is to be rocked by a simulated explosion. Firefighters on the scene will have to determine what fake chemicals or radiological agents are in the air. They will then rush dead-people mannequins and Office of Emergency Management actors made up to look like blast victims out of the arena, first through a decontamination line attended to by people in hazardous material suits, then to a medical tent.
Emergency and military vehicles could be seen at the arena Friday night, along with personnel in HazMat suits. Inside, security personnel actig as victims littered the arena, many with apparent wounds including a pole sticking out of a man's chest and a young fan wounded by shrapnel.
- Emergency Drill Conducted Outside Barclays Center By NYC Officials, Marine Corps - Vosizneias
Don’t duck and cover! The disaster at the Barclays Center tonight is only a drill - Natalie Musumeci & Nathan Tempey - The Brooklyn Paper