SAN FRANCISCO — Four guests at a downtown San Francisco hotel are recovering from carbon monoxide exposure in their rooms.

Authorities say the guests fell ill while staying in separate rooms at the San Francisco Marriott Marquis around 10:40 p.m. Thursday. The guests complained of flu-like symptoms to the hotel's doctor, who urged staff to call the fire department.

Fire Department spokeswoman Lt. Mindy Talmadge says a preliminary investigation found high levels of carbon monoxide that apparently came from the outside through the hotel's air intake system. The incident remains under investigation.

A hotel spokesman says the guests, who work for Singapore Airlines, were taken to the hospital. Three were released early Friday, and the fourth is expected to be released later in the day.