Nassau police said they responded to a report of possible shots fired inside the mall just after 11 a.m.
Authorities say a suspect tried to steal clothing from a Clique store near the food court when he got into a struggle with security.
Police say he had a gun which went off during the struggle. He ran away and got away in a red SUV and was last seen on Mill Road.
Authorities say he was not wearing pants as he got away because he was in the middle of putting clothes on.
"And he got in a red SUV, when he fled he had one sneaker on and he no longer had pants on," Nassau County Police Commissioner Patrick Ryder said. "He was wearing his boxers because he was in the middle of stripping the clothes off that he was attempting to run out of the mall with."
A shell casing was recovered but police believe the suspect took the weapon with him.
He was described as about 5'9" and was wearing a red shirt when he got away. The SUV he got away in did not have a back license plate.
Some of the gates in the mall are automatic so shoppers were locked inside the stores during the lockdown. A nearby school district also went into Lock Out mode.
No injuries were reported.
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