Officials are asking people not to swim at West Haven beaches after sewage and black algae appeared in the water on Wednesday.
Beachgoers saw a "large, dark area" across a mile-long stretch in the water heading toward land around 2 p.m., West Haven Fire Department Chief Robert Schwartz said.
Responding firefighters said the dark area, spanning from Oak Street and Savin Rock, appeared to be sewage and black algae.
Crews are investigating, but said black algae can develop when it's very hot and a boat might have discharged the sewage.
West Haven Health Department, DEEP and the U.S. Coast Guard are asking people to stay out of the water as they investigate and execute tests.