DENVER -- Avian flu has been ruled out as the cause of death of about 375 ducks at a Denver-area wastewater treatment plant, officials said Monday.

The Metro Wastewater Reclamation District said it is still trying to determine what caused the deaths.

Steve Rogowski, the district's director of operations and maintenance, said the state Division of Wildlife ruled out avian flu as the cause.

“The deaths of these ducks continue to puzzle us all,” Rogowski said. “Initially, we found ducks in the chlorine contact basin. However, our ongoing investigation has shown that ducks are dying in locations other than the chlorine basin, as well as on the South Platte River next to our site.”

Scientists walked the South Platte River last week from upstream of the plant at 64th Avenue and York Street to the point where Clear Creek joins the South Platte, a Metro spokesman said. They found a number of dead ducks.

Rogowski said Metro employees have turned over 84 sick or injured ducks to two rehabilitation centers approved by the state and an unspecified number of ducks have been given to wildlife officials for testing at the state's veterinary lab in Fort Collins.

The tests will screen for a variety of diseases and toxicity-related issues.