HANCEVILLE (AP) — Tyson Foods is blaming temporary piping installed by a contractor for pollution that killed tens of thousands of fish in a north Alabama river.

An open letter posted by an arm of the agricultural company says about 220,000 gallons of partially treated wastewater was released from a River Valley Ingredients plant at Hanceville on June 6.

The company says a contractor had installed temporary piping that failed. It says waste reached the Black Warrior River's Mulberry Fork, where an estimated 175,000 fish were killed.

The company says fish died because of low oxygen levels in the water, not chemicals. The company says oxygen levels are now normal and fish are returning.

State conservation officials say it could take years for the river to recover, and area residents have filed suit.