All of those with food poisoning are employees of the Administration’s department for local subsidies on Wednesday, reported Upsala Nya Tidning.
Around 20 people who work with helping councils to supervise food handling at restaurants and other caterers were taking part in the training day.
For dinner they were treated to a buffet of fish, shellfish, cold cuts of meat and vegetables. The next day the first participants fell ill, with vomiting and diarrhoea.
On Friday the environmental health office in Uppsala carried out an inspection of the catering firm which supplied the food.
“We still don’t know what caused these nine people to fall ill,” said Peter Brådenmark, head of the Administration’s supervision department.
“In the evening all of them ate from a buffet which included both fish and meat. There could have been something dodgy in the mussels. But it could also have been caused by poor food handling or refrigeration.”