Information was making its way out about the shocking deaths of 11 adult dolphins in the middle of the night on Monday. The carcasses of the dolphins were discovered alongside the shores of Pass Christian, Miss., a few miles from the Gulf of Mexico.

The East Mississippi Sentinel initially reported that 10 dead dolphins were found and that a fourth dolphin was found along with four children’s sandals that were one foot away from the remains of the pod. The news outlet also said that the source of the calls was a doctor of zoology who believes the pod suffered trauma.

The biologist and marine biologist who contacted authorities, Michael Wise, sent photographs of the shallow graves to Melissa McDonald, of the Environmental Protection Agency’s Fisheries Division. McDonald shared the information with the FBI.

Fish and Wildlife officers were ultimately dispatched and found the remains in shallow ponds and dead trees. The dead dolphins were highly decomposed. The bodies had been there for over a week, the New York Times reported.

In a Facebook post on Tuesday, authorities offered a $20,000 reward for information leading to arrests. By Wednesday morning, they had collected 50 tips. No arrests had been made as of Monday night.

The Guardian reported that the U.S. Coast Guard also patrols the shores of Mississippi and Alabama in search of evidence of illegal dumping.

“If you are caught, we will send you away in a U.S. Coast Guard boat,” the newspaper quoted Lt. Commander Lindsay Ellis, a spokesman for the Coast Guard, as saying.

News of the deaths spurred outrage from residents and others, even as they processed the gruesome discovery. People on Twitter shared photos of the corpses, and expressed their confusion, even as the official cause of death remained unclear. By Wednesday afternoon, there were more than 200 tweets mentioning the matter.

This story has been updated with a statement from the FBI.