The U.S. Centers for Disease Control and Prevention called in DeKalb County health officials this week to try to determine why as many as four dozen employees have contracted a gastrointestinal illness over the past few days.

The employees work in office space in DeKalb County called Century Center that is not part of the main CDC campus, said Tom Skinner, a CDC spokesman. “This is a leased office building; there are no labs or anything other than offices over there,” Skinner said.

About 400 CDC employees report to the building, and many of them work in the CDC’s National Center on Birth Defects and Developmental Disabilities. Skinner said “more than three or four dozen” employees have come down with an illness, with the first wave hitting on Friday.

Reports of illness continued over the weekend and into this week, prompting the CDC to allow the building’s healthy employees to work from home to avoid exposure.

CDC cleaned the building and had the water tested. Skinner said late Wednesday that water samples came back showing no contamination. But the agency planned to keep the building closed until Monday and encouraged employees to work from home until then.

CDC is working with DeKalb County to try to identify what may have caused the outbreak.

“We do not know what it is. It is behaving like a norovirus, but we do not know exactly what is causing the illness yet,” Skinner said.

Skinner said DeKalb health officials hope to gather specimens from some of the sick workers to try to identify the cause.

Norovirus is a very contagious virus that can cause stomach pain, nausea, vomiting and diarrhea, according to the CDC’s website. Norovirus is best known for causing outbreaks on cruise ships, but it can also quickly make the rounds at schools, nursing homes and other locations where groups of people occupy relatively small spaces.

Since this off-campus office does not house any CDC labs and workers aren’t involved in investigating infectious diseases, there was no immediate indication that the outbreak resulted from the CDC’s own work on infectious diseases.

“At this point it’s too early to tell exactly what is causing the illness,” Skinner said.