The deaths of 90 dolphins along the Gulf Coast this year have spurred a federal investigation and allegations from green groups linking the strandings to last summer's massive Gulf of Mexico oil spill and the heavy use of chemical dispersants in the cleanup.

Most striking, researchers say, is a spike in the numbers of dead newborn, premature or stillborn dolphins -- 47 this year, the National Oceanic and Atmospheric Administration said. This compares with the annual average of nine in the first three months, the birthing season for dolphins in the Gulf, the agency said.

The cause of the deaths has yet to be determined, but some researchers have pointed to a steep drop in water temperatures caused by unusually heavy snowfall in Alabama this winter, while environmentalists maintain the culprit could be poisoning related to the oil spill.

"We're still in the response phase of the investigation," said Blair Mase, a NOAA marine mammal specialist. "Our teams are conducting necropsies as soon as the animals come in." A necropsy is a postmortem exam on an animal.

Getting results from the investigation, Mase noted, could take months, and the decomposed state of the dolphin carcasses when they wash ashore could make it difficult to crack the case.

NOAA also noted that unusually high numbers of dolphin strandings in the Gulf began in February 2010. There have been three peaks -- last February, over the summer last year and the one occurring currently.

Research by the Dauphin Island Sea Lab in Alabama has suggested that unusually cold water in the Gulf has had a hand in the deaths.

William "Monty" Graham, the lab's senior marine scientist, said that an unusually large snowfall in a January storm corresponded with two peaks in dolphin strandings.

The first spike came shortly after the snowfall, and the second and more significant pulse took place two weeks later, when large amounts of cold-water runoff flooded Mobile Bay and caused water temperatures to drop 15 degrees.

"Two big pulses of cold water both coincided with the immediate strandings of dolphins," Graham said in a telephone interview.

If cold water was the cause, that does not rule out the oil spill's playing a role in the strandings, Graham said.

"People are trying to shoehorn this into a toxicity issue, when toxicity might not show up in the necropsies," he said. "The question ought to be, why couldn't those animals get out of the way of the cold water?"

To determine that, Graham recommended studying dolphins' blubber and fat levels. If the dolphins -- and particularly mother dolphins -- were malnourished because they could not eat properly following the oil spill, they may have been susceptible to disease, and that might explain the strandings.

Enviro groups target spill, NOAA

Several environmental groups are focusing their sights on NOAA, saying the agency's probe needs to be more transparent. Their suspicion is that the dolphin deaths are connected to the spill and to chemical dispersants used in the cleanup.

"We don't have CSI for marine mammals," said Aaron Viles of the nonprofit Gulf Restoration Network. "There are a myriad of different ways that the historic release of oil and the historic amount of toxic dispersants were interacting with the ecosystem in an unquantifiable way. When you see a spike like this, it's almost guilty until proven innocent."

What NOAA's necropsies rule out as the cause of death will be as important as what they rule in, according to some groups. Michael Jasny, a senior policy analyst with the National Resources Defense Council who has closely followed the dolphin strandings, echoed Graham's remarks in noting that it will be very difficult to pinpoint oil or the dispersants as the cause of the dolphin deaths.

"It's important for the public to understand the importance of that question mark," Jasny said. "If NOAA is able to rule out causes like red tide and other common sources of infection, it really should raise the specter of the spill being responsible."

Others are not shying away from focusing criticism directly at NOAA.

"One of my biggest concerns with the statements we've seen from NOAA and other agencies has been that the scope of investigations [into the Gulf spill] has tended to be too narrow and the conclusions they are making are too broad," said John Hocevar, the director of Greenpeace's oceans campaign.

But experts cautioned strongly against attributing the cause of the dolphin deaths to the oil spill.

Joseph Geraci, a professor in the Department of Pathology at the University of Maryland's School of Medicine, has written a book on dolphin strandings and led a federal investigation into a massive die-off of dolphins along the East Coast in the late 1980s. He said that while he "wouldn't rule anything out," there is "nothing pointing to oil."

Geraci suggested that other causes could include a virus contracted by the mothers of the dead dolphins, or some sort of toxic affecting marine plants.

If the necropsies of the stranded animals do find a link to the oil spill, it could pave the way to legal action against the company whose well spewed the oil, BP PLC, Geraci added. In that case, it may take even longer for the results to become public.

"It's frustrating," he said. "But I understand why the study has to be done in such a guarded way."

Any delay or lack of transparency will only anger environmental groups, which are calling for increased funding for the analyses as well as new studies on the long-term effects of the oil spill on marine life in the Gulf.

Naomi Rose, a marine mammal specialist with the Humane Society of the United States, said that studies of the 1989 Exxon Valdez spill in Alaska found lasting effects on a group of orcas. In the years following the spill, "the number of animals in that pod declined almost to the point of extinction."

Rose said orcas showed that simple environmental exposures had a lasting -- and devastating -- impact on the group.

"It's not that you'll find oil in their bodies," Rose said. "It could be something as simple as they breathed in the oil as they moved through it."