California health authorities have determined that the highly contagious norovirus was responsible for sickening 60 people at an upscale Bay Area hotel, sending about a dozen to the hospital, officials said on Monday.

San Mateo County Health System spokeswoman Robyn Thaw said norovirus was identified as the source of an outbreak of gastrointestinal illness among guests who attended an NAACP conference and two other events at the Hotel Sofitel in the San Francisco suburb of Redwood City in late October and early November.

"Specimens have been collected and tested across three separate events held at the hotel," Thaw said in an email. She said 60 hotel guests and employees became sick but did not have a breakdown of the numbers that fell ill at each event.

She said samples were being sent to state health authorities to determine if the norovirus cases among members of the U.S. civil rights group were linked to illnesses at the other events.

At least 12 people from the hotel were taken to the hospital by ambulance on Oct. 26, the day some guests at the NAACP meeting first became ill, along with some others who drove themselves to local emergency rooms, Thaw said.

Thaw said she did not know whether any of the patients ended up hospitalized, adding, "We know patients were treated for dehydration and nausea and released within several hours."

Norovirus is the most common cause of acute gastroenteritis in the United States, with up to 12 million illnesses each year resulting in up to 71,000 hospitalizations and about 800 deaths, according to the San Mateo County Health System.

Health officials said an initial onsite inspection of the food operation found no violations linked to food-borne illness, and the hotel resumed its food service last week after shutting it down in response to the outbreak.

"We're making sure that ... every possible attention to hygiene is taken care of," hotel spokeswoman Sandra Duhamel said, adding that the hotel had told incoming guests of the situation.

The California Department of Public Health and San Mateo County health officials were also interviewing people who reported being ill, to learn more about how the outbreak started and spread.