The smell of gas wafted through Center City on Monday, briefly leading to the evacuation of multiple buildings and prompting an hours-long investigation into its source.

The cause of the odor remains undetermined, the Philadelphia Fire Department said late Monday afternoon.

The Philadelphia Office of Emergency Management first said the release of mercaptan, which it called a non-hazardous substance, during equipment cleaning at the shuttered Philadelphia Energy Solutions refinery led to the citywide reports of an odor.

But the Philadelphia Fire Department later said it could not confirm that initial report, though the refinery site did clean equipment Monday.

“We don’t know what the cause of these odors is," Fire Commissioner Adam Thiel said.

The refinery had a reading on its monitoring system about 7:30 or 8 a.m. that suggested “some type of product went across their facility from an unknown location,” Thiel said. But the Fire Department checked with the New Jersey Department of Environmental Protection and there were no reported releases from other facilities.

Thiel first became aware of the odor when the Municipal Services Building was evacuated. When the Fire Department responded to that location, it did not find anything.

“With the atmospheric monitoring equipment that we have available, they did not find any hazards. Frankly, that does not mean there are not hazards there,” Thiel said. “There is a limit to what we are capable of finding with the equipment that we have, and of course, the atmospheric conditions are constantly changing. It’s a windy day today.”

Thiel said he did not know how many odor reports the Fire Department received because there were “so many,” but said there were upward of 500 emergency calls to the Philadelphia Gas Works reporting the odor of gas. PGW said a natural gas leak was not the source of the “reports of foreign gas odors across the city of Philadelphia.”

Sophie Kluthe, an American Red Cross spokesperson, said Red Cross staffers were evacuated from their offices at 22nd and Chestnut Streets about 11:20 a.m. and allowed back in about 10 minutes later.

“We walked out of the building, the fire trucks pulled up, and then they were talking to my bosses, and all of a sudden we were allowed back in,” she said.

The Public Interest Law Center closed its office in Center City “due to a suspected gas leak.”

At the Criminal Justice Center, an announcement over the loudspeaker attributed the odor to an additive used in cleaning the refinery in South Philly. The announcement said it was not natural gas and was not harmful.

In City Hall, an announcement also said an odor was emitted through the air at the refinery: “We are all clear, no gas leak, you can return to work."

Staff writers Chris Palmer and Laura McCrystal contributed to this article.