New laboratory test results are providing little insight into the origin of a salmonella outbreak linked to an east Vancouver restaurant. Meanwhile, public health officials report their investigation costs have reached $44,000.After shutting down the On the Border restaurant on Oct. 9, county health inspectors spent the day taking samples from surfaces throughout the restaurant to try to determine the source of the outbreak. Dr. Alan Melnick, Clark County health officer, said Wednesday those tests all came back negative for salmonella.Some of the surfaces had been cleaned by restaurant staff prior to health inspectors retrieving samples. That means the source could have been cleaned, or the bacteria was spreading by another method, such as a sick food handler or tainted food, he said.“We’re never going to be able to say definitively what caused this,” Melnick said. “I wish we could all feel better with a definitive answer as to the cause.”The restaurant’s management did not respond to messages left Wednesday by The Columbian.Melnick said pinpointing the cause of food-borne illness outbreaks is difficult and rare.The restaurant has been linked to 117 confirmed and probable cases of salmonella as of Wednesday morning. Public health staff interviewed those who got sick and, in some cases, their dining companions who didn’t. Those interviews and a statistical analysis revealed a handful of food items in common among people who got ill, Melnick said.