Douglas County Public Health is continuing its investigation into what caused Fir Grove Elementary School students and teachers to become ill Thursday with stomach flu-like symptoms. The source of the illness has still not been determined, but officials said the health department has collected a number of specimens and expect results from lab tests this week. The department also conducted an assessment of Fir Grove Elementary and provided recommendations to the school on cleaning procedures. Health officials say families that have someone who is sick with a stomach bug should reserve one bathroom for that person if possible. After nearly …