Police responding to a call about an injured woman at the Legal Seafood restaurant in Huntington Station, New York, found the 55-year-old manager unconscious in the basement, Suffolk County Police said.
The manager, Steven Nelson, died of cardiac arrest as he was being transferred by ambulance said Julie Robinson-Tingue, a spokeswoman for Huntington Hospital.
Soon after arriving, police and emergency personnel felt nauseous and dizzy and recognized the symptoms as a carbon monoxide event, Suffolk County police said.
At least three people taken to the hospital remained in a stable condition while at least six were treated and released, she said.
In all, 27 people were taken to hospital, according to Suffolk County police.
Most victims were restaurant employees, but three police officers and four ambulance employees were among those who fell ill, Suffolk police said.
Homicide and arson squad detectives were investigating, with focus on the restaurant's heating system.
Neither representatives for the mall nor the restaurant could be reached for comment.