Federal scientists are trying to determine why an extraordinary number of dolphins have turned up dead along the Gulf Coast.

A vast majority of the 279 common bottlenose dolphins discovered stranded along the Gulf in Florida, Alabama, Mississippi and Louisiana since Feb. 1 have died, officials with the National Oceanic and Atmospheric Administration said Friday.

The number of deaths is about three times the normal stranding rate for the time period, NOAA officials said. About 78 percent of the carcasses were too decomposed for study, they said.

Suspects so far include the Deepwater Horizon oil spill of 2010, which has had a lasting impact on wildlife along the Gulf shore, and spring's historic Midwest river flooding, which has sent freshwater rushing into the Gulf.

Teri Rowles, coordinator for NOAA Fisheries' Marine Mammal Health and Stranding Response Program, said the stranding was taking place in "some of the exact same areas" affected by the spill.

"[Dolphin] reproduction in some of the heaviest oiled areas continues to be abnormal," she said.

At the same time, scientists studying dolphin carcasses have discovered "visible skin lesions consistent with freshwater exposure," said Erin Fougères, administrator for the marine mammal stranding program in NOAA Fisheries' southeast region.

Nearly one of four stranded dolphins examined by scientists exhibited signs of freshwater exposure, which could be related to the abnormal river flooding, the NOAA scientists said.

They said Monday that 7,829 square miles of land and Gulf coastline were forecast to be part of an annual wildlife "dead zone" this summer because of seasonal river flows that create low-oxygen conditions underwater.

The cause of what NOAA is calling an "unusual mortality event" for dolphins remains a matter for scientific inquiry, and officials said they were creating an investigation team to get to the bottom of the deaths.

Scientists asked boaters and beachgoers who come across stranded or dead dolphins along the Gulf Coast to stay away and notify authorities, such as the U.S. Coast Guard, immediately.