National Grid was on site first thing Monday morning, cutting off gas service to Legal Sea Foods from the street. It's unclear when the restaurant will open again.
Huntington Chief Fire Marshal Terence McNally says the fumes were circulated in the basement by the ventilation systems.
McNally said Sunday the restaurant was inspected last March, and no problems found. Another inspection was scheduled for next month.
The company has been issued a citation for defective equipment. McNally says the flue pipe will have to be repaired before the restaurant can re-open.
Dangerously high levels of carbon monoxide were detected Saturday night inside the mall.
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Suffolk County police responded to the mall just after 6 p.m. after a worker collapsed and fell down a flight of stairs at the Legal Seafoods restaurant.
Police believe the worker was overcome by the gas and collapsed. Manager Steven Nelson, 55, was found unconscious in the basement. Police tried to resuscitate him, but were unsuccessful.
By the time crews found him, it was too late. Friends say he was healthy and always exercised, and was caring, wonderful and always helping everybody.
Also inside the restaurant was Assistant Manager Megan Smith, who rushed to make sure everyone was safe and also briefly succumbed to the lethal fumes. Smith is being called one of the many heroes. During the effort to get her out after she collapsed, she was heard saying, "You know, where's Steve? Is Steve all right?"
Nelson and Smith were both rushed to Huntington Hospital, where Nelson was pronounced dead on arrival. Smith is in critical condition.
For Legal Seafoods Owner Roger Berkowitz, it wasn't easy walking back into his restaurant where time stood still, and everything was left exactly where it was when officials ordered everything out of the building when carbon monoxide had taken over.
All of those affected by the carbon monoxide were restaurant employees, police or ambulance workers, said Suffolk County Police Lt. Jack Fitzpatrick.
"Right now, we are inspecting the heating system, and this incident seems to be confined to the basement area," Fitzpatrick said. "It does not appear to have made it in the area of the restaurant where the customers were."
Legal Seafoods, Panera Bread and the Cheesecake Factory were evacuated for precautionary reasons. But the mall, which is not structurally connected to the restaurant, remained open.
"When I came out, there was a whole bunch of ambulances everywhere, and they were telling everybody to come and get checked out," employee Evelyn Toloza said. "It was just bizarre."
"They told us to leave because of a gas leak," Cheesecake Factory patron Kathy Sella said. "I didn't want to blow up or anything like that. We were at the bar having a glass of wine and then, one of the waitresses, she said you have to leave."
Carbon monoxide is odorless and colorless and can lead to death by suffocation.
Most people are unaware that carbon monoxide detectors are not required in public properties.
Suffolk County Legislator John Kennedy Jr. says that needs to change and he plans to start with all county office buildings and community college facilities.
"We own them, we have control over them, we have the ability to take the affirmative action," said Legislator John Kennedy, Jr., (R) Suffolk County.
Kennedy says the next step will be trying to force all commercial and business properties throughout the state to have carbon monoxide detectors.
But he says he's putting together a task force to take a look at it.
"As a Republican I am no fan of mandates. However, who can put a value of a life. We have to make sure that we have public spaces that are safe," Kennedy said.
The Walt Whitman Mall consists of more than 80 stores, including anchors Bloomingdale's, Lord & Taylor, Macy's and Saks Fifth Avenue. The restaurant is closed Sunday.
In a tweet, Legal Seafoods said, "We are profoundly saddened to learn of the tragic death of our General Manager Steve Nelson. Our thoughts and prayers are with his family."
The Associated Press contributed to this report.