The man who disarmed the Monterey Park killer -- likely preventing a second mass shooting over the weekend -- is speaking out about the terrifying altercation ... saying something just came over him as he saw a firearm, springing him into action.
Brandon Tsay, who helps run the Lai Lai Ballroom & Studio in Alhambra, CA with his family, told 'GMA's Robin Roberts he heard the front door close behind him Saturday night during their Lunar New Year celebration ... which is when he saw 72-year-old Huu Can Tran standing inside with a gun.
He goes on to detail his brave struggle with Tran ... adding he got bashed in the back of the head, eventually using his elbows to separate Tran from the gun -- shouting at him to "Get the hell out of here" while pointing the weapon at him.
Tsay says he didn't run off immediately, but instead oddly stuck around before jogging back to his van.
As you know, Tran tragically took the lives of 10 people and harmed another 10 at a separate festival at Star Ballroom Dance Studio in Monterey Park -- not far from Tsay's ballroom. Law enforcement surrounded his van Sunday in Torrance, where police later confirmed he died by suicide inside.
Tran frequented the Monterey Park ballroom in the past ... with people who knew him telling CNN he even met his ex-wife there -- and reportedly believed the instructors had said "evil things about him."
Police and investigators are still working on a motive for the attacks.