So they were shocked when an onsite necropsy showed no signs of oil-related injury, or of bacterial infection, biotoxins, or disease—the most common causes of death in dolphins.

Instead, the Louisiana officials found a tiny piercing on the right side of the dolphin's blowhole. That hole would later reveal the cause of death: a small bullet lodged in the animal's lung.

The killing turned out to be another in a growing string of apparent attacks on dolphins and other marine mammals reported along the Gulf Coast in recent months.