Mystery toxin suspected as 100 tons of dead fish removed from Oder River on German-Polish border

Berlin — A river on the German-Polish border has become a mass grave. Volunteers and authorities have been busy for days removing tons of dead fish floating on the surface of the Oder River.

The cause of the massive die-off, which has affected a stretch of the river more than 300 miles long, remains a mystery, but Sascha Maier, a water policy officer at the BUND environmental organization, told CBS News on Wednesday that "everything points to a manmade catastrophe."

Authorities are struggling to find answers.