Emergency crews rushed to the mall in Huntington Station in New York following reports that a woman collapsed in the basement of Legal Sea Foods.
Once they arrived, they felt dizzy as well, and determined it was carbon monoxide poisoning, Suffolk County Police said in a statement.
"Police evacuated the restaurant and found the manager, Steven Nelson, 55, of Copiague, unconscious in the basement," the statement said.
"He was transported to Huntington Hospital where he was pronounced dead."
Surrounding businesses were evacuated as a precaution, but authorities said the carbon monoxide appeared limited to the basement of the restaurant.
The investigation is focusing on the restaurant's heating equipment, authorities said.