OVERLAND PARK, Kan. (AP) — The number of people sickened in a norovirus outbreak linked to a suburban Kansas City dinner theater has grown to nearly 400.

The Kansas Department of Health and Environment said in a news release Thursday that the people who reported contracting the gastrointestinal illness attended performances at the New Theater Restaurant in Overland Park starting on Jan 15.

Four people have laboratory specimens to confirm norovirus. The theater holds about 625 people

Health officials are asking people who have attended a performance since Jan. 15 to participate in an online survey. The source of the illness is under investigation.

Rob McGraw, vice president of sales and marketing for the theater, told The Kansas City Star that it’s his understanding that only three employees have been confirmed for norovirus.