n">(Reuters) - California health authorities have determined that the highly contagious Norovirus was responsible for sickening 60 people who attended an NAACP conference last month 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 the gastrointestinal illness had been suspected of sickening guests who attended three events at the Hotel Sofitel in the San Francisco suburb of Redwood City in late October.

"Specimens have been collected and tested across three separate events held at the hotel," Thaw said in an email, without giving details of the events or saying whether all the events were for the U.S. civil rights group.

  "Norovirus was identified in several specimens collected and samples have been sent to California Department of Public Health for additional testing to determine if the Norovirus has a common cause across the three hotel events," she added.

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 foodborne illness, and the hotel resumed its food service last week after shutting it down in response to the outbreak.

"We are making sure that we're doing everything and anything possible to adhere to every possible attention to hygiene is taken care of," hotel spokeswoman Sandra Duhamel said, adding that the hotel had notified 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.

(Editing by Cynthia Johnston and Eric Walsh)