A chemical byproduct found in some treated drinking water in the U.S. that remained a mystery for more than 40 years may have finally been identified by scientists. The authors of the study, published Thursday in the journal Science,

Scientists have identified the byproduct of a chemical used to disinfect tap water and prevent waterborne illnesses. Researchers urge testing for toxicity.

About a third of U.S. residents have been receiving tap water containing a previously unidentified chemical byproduct, a new study has found. Some scientists are now concerned — and actively investigating — whether that chemical could be toxic.