A massive fish kill spanning at least 20 miles along Georgia’s Chattahoochee River is under investigation after thousands of dead fish were discovered.

Environmental groups say it is the worst such event seen in decades, with possible links to flooding and low dissolved oxygen levels in the water.

Experts suggest that runoff or treated wastewater with low oxygen may have contributed to conditions that suffocated aquatic life.

Officials from state and city agencies are collecting samples and have not yet confirmed the cause, the Independent has reported.

Early test results are expected within days, while authorities say human health risks appear low but caution against river use in affected areas.