NEW YORK (CBSNewYork) — At least nine people were injured after a carbon monoxide leak at a construction site in Manhattan Tuesday evening.

FDNY members rushed to the site at 30 E. 29th St. in Kips Bay around 4:45 p.m. after reports of a leak.

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Several workers were rescued from a confined space at the construction site, according to authorities.

“We connected some ropes and some hardware to that, and were able to put the patients in a basket and haul them up,” FDNY Chief of Special Operations John Esposito said.

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Officials say the leak came from a gas powered generator in a basement on site where they were preparing to pour concrete.

“You’re not supposed to be using generators,” Esposito said. “They’re supposed to be outside in a well-ventilated area.”

The city’s Department of Buildings says there were two of those generators in the basement, and they’ve since issued a full stop-work order. They’ve also slapped the site’s contractor and management team with several violations for failing to keep things safe.

The DOB issued a permit for the residential building construction in September, and last inspected it a few weeks ago but found no problems.

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Five of the workers sustained minor injuries, while four of them were considered to be in serious condition according to the FDNY.