The Nashville Fire Department responded after a carbon monoxide alarm sounded in one apartment. Crews also checked five nearby buildings and detected elevated natural gas levels in four. Building H was the only one under a mandatory evacuation order; residents in the other buildings were allowed to leave voluntarily, and many did.

Fire officials said Spire, the natural gas utility, determined the readings were not caused by a leak from the commercial gas supply. Investigators said a construction site uphill from the apartments has multiple holes from drilling or blasting, and that work may have disturbed gases underground. The cause had not been confirmed.

Because the gas was not coming from the commercial supply, it lacked the odorant commonly added to natural gas to produce a rotten-egg smell, the fire department said. Officials warned that the gases may be odorless and can pose health risks at certain concentrations.

The fire department said Saturday afternoon that Building H would not be cleared for occupancy that day. Crews planned to maintain a reduced presence and periodically monitor conditions while the issue was addressed. Later, the department said it was handing responsibility for securing the building to property management and coordinating with construction crews on a safety plan before work resumed.

No injuries had been reported.