The Environmental Protection Agency sent an emergency team to South Dakota over the weekend after the Flandreau Santee Sioux Tribe reported a massive fish kill in the Big Sioux River.

Anywhere from 30,000 to 50,000 fish died last week, according to the EPA. Possible causes include a natural winter kill or phosphates from arm fertilizers. Tribal officials say the large number, location -- most natural kills occur in lakes, according to the tribe's water quality specialist -- and varying species of dead fish were troublesome.

The EPA took samples of the water. Results are due next week.

Get the Story:
EPA, tribe probe huge foam heap in Big Sioux (AP 3/3)
Thousands of fish die in river (The Sioux Falls Argus Leader 3/2)