The U.S. National Whitewater Center voluntarily closed its whitewater rafting activities Friday afternoon after water samples tested positive for a brain-eating amoeba, officials confirmed.

The tests, which found Naegleria Fowleri at the Whitewater Center, are preliminary. Final results will not be ready until next week.

Visitors said a staff member blew a whistle around 4 p.m. Friday and had everyone get out of the water. The center itself, in Charlotte, North Carolina, is not closed, however, Mecklenburg County Health Department Director Dr. Marcus Plescia said officials "feel quite certain (the) amoeba is present in (the) Whitewater Center" at a news conference Friday evening.

Local, state and federal health officials are investigating after a recent high school graduate, Ohio resident Lauren Seitz, 18, died from a brain-eating amoeba days after visiting the Whitewater Center.

The Health Department is working closely with the Center.

Naegleria Fowleri is a brain-eating amoeba found in warm freshwater, like lakes and ponds. It does not make people sick if it's swallowed, but if it goes up a nasal cavity -- where it's close to the brain -- it can be deadly. The amoeba is present in many open water sources and has been linked to 35 illnesses in the last 10 years, Plescia said.

He said the risk of getting into a car crash while driving to the Whitewater Center is higher than being infected by the amoeba at the facility.

Plescia said the Centers for Disease Control and Prevention was running tests Wednesday at the facility. The health organization took 11 samples from various parts of the Whitewater Center and found the amoeba in most of them. Testing will continue over the next few days.

The Center released a statement:

"The U.S. National Whitewater Center, after discussion with the Centers for Disease Control and local health officials, has decided to temporarily suspend all whitewater activities effective immediately.

This decision was made after initial test results found Naegleria Fowleri DNA was present in the whitewater system.

The USNWC is working with the CDC and local health officials to develop next steps. Only whitewater activities are suspended.

The USNWC remains open for all other operations and activities."