Sen. Bill Nelson, D-Fla., on Wednesday revealed chilling details about how the shooter suspected of killing at least 17 people at a high school in Parkland, Fla., brought a gas mask and smoke grenades with him in preparation for the attack.

"He wore a gas mask and smoke grenades. He set off the fire alarm so the kids would come out into the hallways. And thus, he had the opportunity with a crowded hallway to start picking off people," Nelson told MSNBC.

Nelson later told CNN the FBI had not confirmed with him that any of the smoke grenades had been set off.

Broward County Sheriff Scott Israel identified the shooting suspect as Nikolas Cruz, 19, who had been previously expelled from Marjory Stoneman Douglas High School, where the shooting took place.

Students and adults are among the victims, though Israel could not confirm whether any were teachers.

Cruz was also brought to the hospital, but was later released into police custody.