Earlier this morning, the San Francisco fire department said the carbon monoxide was getting into the hotel through the heating system. Now they believe it could have been car exhaust that got in through the ventilation system.
Four people got sick at the Marriott Hotel just before 11 last night - all with dizziness and nausea. A blood test showed carbon monoxide poisoning, but the source was and still is uncertain - though firefighters believe they know.
"The thinking right now is that it came in through?" asked ABC7's Terry McSweeney.
"Through the air intake system and it basically just sucks the outside air in and then it goes through their ventilation system," said Mindy Talmadge of the San Francisco fire department.
It's the third time this week fire crews have been at the hotel for carbon monoxide problems.
"January 4 we responded to two similar calls, at two separate times, to those same two rooms," said Talmadge.
The affected rooms are right above Stevenson Street, where trucks and cars park, sometimes leaving their engines running. It's possible the exhaust is what got into the rooms. But no other rooms were affected - not last time and not this time. It's still a mystery and a disturbing one to some hotel guests.
"I'm nervous, but I talked to the guy over at the desk and he said they are getting calls every five minutes and they are checking the whole hotel. He tried to reassure me, but I'm going to ask again when I come back," said Boston resident Marge Dolby.
"They said it was contained to the fourth floor and it's all taken care of for now. That is all I heard, " said hotel guest John Dolby.
The San Francisco Marriott Marquis released a statement: "The situation continues to be under investigation and we are awaiting further information regarding the source of the carbon monoxide exposure. Meanwhile the four guest rooms will remain out of inventory until the situation has been resolved."
There are no carbon monoxide detectors in the affected hotel rooms, by law there does not need to be. According to the San Francisco fire department, new construction will require carbon monoxide detectors.