DENVER -- Wildlife officials are investigating the deaths of some 200 ducks in a chlorine basin at the Denver metro wastewater facility.

All of the ducks were found dead in the basin over the past two weeks. The basin is the final stage of treatment before the water goes into the river.

Steve Frank of Metro Wastewater Reclamation District said it is an unusual occurrence and has not happened at the facility in 20 years.

"It's not unusual at all to have ducks here. The ducks come every winter, but we just don't see ducks die," Frank said.

Security officers said they are firing blank shots every half hour to try to scare the birds away. Staff members plan to cover the basin to keep the birds out.

State and federal wildlife officials were investigating, and lab tests were being performed in Fort Collins to determine a cause.

The plant increased the amount of chlorine in the water in December, but Frank said employees did not see any change in the number of ducks, dead or alive.