ATLANTA — An investigation into the mysterious killing of dolphins in the Gulf of Mexico is gaining momentum, with a federal agent now in charge of the case and a $30,000 reward for catching the killer or killers.

Since the summer, six dead dolphins have washed ashore with bullet wounds, severed fins and jaws, and stab wounds. The latest was a dolphin found off the coast of Mississippi with its lower jaw missing.

Whether the deaths were caused by the same group or person is not known, but federal investigators are turning to the public for help.

The National Oceanic and Atmospheric Administration has called on fishermen and beachgoers to report any dead or injured dolphins. Forensic experts at the agency are testing the slugs removed from the animals to see if they come from the same gun.