Carbon monoxide that sent four San Francisco Marriott Marquis guests to the hospital Thursday night may have come from a leaking water heater located near the hotel, a fire spokeswoman said Friday afternoon.

Firefighters had responded at about 10:40 p.m. Thursday to reports of multiple people suffering from flu-like symptoms at the hotel, located at the intersection of Fourth and Mission streets, San Francisco fire Lt. Mindy Talmadge said.

Four guests were transported to San Francisco General Hospital and St. Francis Memorial Hospital to be treated for poisoning due to high levels of carbon monoxide measured in two rooms at the hotel.

The potentially deadly gas apparently got into the rooms through the hotel’s air intake system, which draws air in from the outside, Talmadge said.

Three of the four guests were released from the hospital, and were back at the Marriott Marquis in new rooms Friday morning, hotel spokesman Sam Singer said. The fourth guest was expected to be released later in the afternoon, Singer said.

The carbon monoxide apparently came from a leaking water heater that supplied a restaurant near the hotel, Talmadge said.

The heater had work done on it recently, she said.

Investigators were still gathering information on the incident Friday afternoon, she said.

Thursday’s incident may not have been the first carbon monoxide poisoning at the hotel this week.

Ambulances had also responded to the hotel on Tuesday evening for two separate reports of guests suffering from similar flu-like symptoms, Talmadge said.

Those guests were staying in the same rooms that were affected Thursday, but “there were no red flags to indicate carbon monoxide poisoning on Tuesday,” she said.