The 2010 BP oil spill in the Gulf of Mexico caused an unprecedented number of fatal diseases in roughly 1,300 dolphins over the course of five years, according to a new study by the National Oceanic and Atmospheric Administration. Up to now, the link between oil spill exposure and dolphin deaths has been inconclusive, but this study changes that. The results are from a forensic investigation that was part of NOAA’s long-term ecological analysis of the Deepwater incident that began in 2013. The spill itself leaked 4.9 million barrels of oil into the Gulf over five months in 2010. “No feasible alternatives remain that can reasonably explain the timing, location and nature of this increase in death,” co-author Stephanie Venn-Watson of the National Marine Mammal Foundation in San Diego said in a Wednesday press conference.

BP has always challenged the claim linking the oil spill to dolphin deaths, but the study examined unusual mortality events (UMEs) in which large numbers of dolphins perish in a short time frame and from similar causes. Afterwards, researchers drew firm conclusions about the die-offs. The NOAA team examined the major organs of 46 dolphins that had died along the coasts of Louisiana, Mississippi and Alabama from June 2010 to December 2012. Researchers then compared results to a control group of 106 dolphins that had washed up off the coasts of the Carolinas between 1996 to 2012, or those found off the coasts of Florida and Texas prior to the Deepwater oil spill. FULL REPORT