Huntington Chief Fire Marshal Terence McNally said the fumes were circulated in the basement by the ventilation systems at the Legal Sea Foods restaurant at the Walt Whitman Shops on Long Island.
Restaurant manager Steven Nelson was found unresponsive in the basement on Saturday night and pronounced dead at a hospital.
Roger Berkowitz, president and CEO of Legal Sea Foods, said Nelson, 55, of Copiague, had worked for the restaurant for three years and had two sons.
"It's a shock, he was a great guy, we consider ourselves a family," Berkowitz said, adding that other employees were "traumatized."
Berkowitz said the carbon monoxide leak was "a wakeup call for commercial businesses" and that monitors should be in all businesses.
Authorities initially went to the restaurant after receiving a call about a woman who had fallen and hit her head in the basement. Rescue workers who arrived at the scene started to feel lightheaded and nauseated and suspected a carbon monoxide leak, officials said.
The restaurant was evacuated and 27 people were treated at hospitals. All of those impacted by the fumes were restaurant employees, police or ambulance workers.
The building was not required to have carbon monoxide detectors, and there were none, McNally said.
Carbon monoxide is odorless and colorless and can lead to death by suffocation.
Police said a coroner would officially determine Nelson's cause of death. Autopsy information was not immediately available Sunday.
No problems had been found when the restaurant was inspected last March, and another inspection was scheduled for next month, McNally said.
The Walt Whitman Shops, located about 35 miles east of New York City, has more than 80 stores, including Bloomingdale's, Lord & Taylor, Macy's and Saks Fifth Avenue. The restaurant is in a separate structure from the main group of stores, in a block with The Cheesecake Factory and Panera Bread. Both restaurants were evacuated as a precaution, and The Cheesecake Factory was open as usual on Sunday.
Gas lines to Panera Bread and Legal Sea Food had been shut off, McNally said, with Panera likely to get gas back Monday morning. Legal Sea Foods, which was issued a citation for violating town ordinances by having defective equipment, will have to show officials that the water heaters have been repaired before it can re-open, he said.