But Barry-Eaton District Health Department officials say they are still trying to pinpoint the cause.
"We are still investigating to get a clearer picture," said Eric Pessell, the department's director of environmental health.
In a report today, the health department will cite the restaurant for violations in two critical areas: allowing at least one sick person to work and for improper hand-washing practices.
"We have found at least one employee - if not up to three or four employees - who were symptomatic and working," Pessell said.
The employees were either involved in food preparation or were servers, he said.
The employees worked between Jan. 27 and Jan. 30.
Officials say the vast majority of reports are from people who ate at the restaurant on Jan. 28 and Jan. 29.
The health code states that sick employees have to inform management if they have certain symptoms.
"That may or may not have happened," Pessell said, adding that an employee being sick is the leading cause of food-borne illness in the United States.
The restaurant requires employees to agree that they won't work when they're sick, Pessell said.
Messages left with Outback Steakhouse Inc., which owns Carrabba's Italian Grill, were not returned.
The number of people reporting that they became ill after eating at the restaurant continues to rise, with health department officials on Monday putting the total at 437.
Attorney Drew Falkenstein of Marler Clark in Seattle, a firm that has handled dozens of cases of norovirus outbreaks at restaurants, called the Carrabba's outbreak the largest at a restaurant he has ever seen.
"Typically, we see cases where 10, 20, 30 people are affected," he said. "A big outbreak is 60."
Dr. Robert Schirmer, the health department's medical director, said publicity has been a factor. People are hearing about the outbreak and making the association between symptoms they had and eating at the restaurant.
"It's kind of the 'Aha!' phenomenon," he said.
Health department officials met with the restaurant's management Monday to put together a plan to address the violations.
The illness was caused by a norovirus. Norovirus outbreaks are typically caused when an infected person handles food in an unsanitary manner.
Symptoms include severe vomiting and diarrhea that usually last about a day.
Two health department inspectors were at the restaurant Jan. 31 through Thursday, Pessell said. Since then, the department has been in constant contact with the restaurant.
"We will continue to go out to the facility in a more routine nature than we would have had this not happened," he said.
The restaurant, like most, had been cited for critical violations in the past one or two years, but not for allowing a sick employee to work, Pessell said.
Health inspectors, on average, find one or two violations per inspection, he said. Carrabba's would normally be inspected three times a year, he said.
"Over the next couple of months, our presence will be known," he said.