The die-off began five months ago.

Freshwater turtles began turning up dead along the St. Johns River in January. Now about 100 dead or dying turtles have been found in water bodies in Orange, Seminole and Putnam counties. A few reports have come in from other locations, such as Trout Lake near Eustis in Lake County.

Examinations of the turtles and tests of their tissues have, at this point, failed to pinpoint a cause of death, according to the Florida Fish and Wildlife Conservation Commission. There were no obvious signs of injury.

Now the commission has officially opened an investigation, in collaboration with the University of Florida, and is asking the public for help. Anyone who finds a dead turtle should contact the agency's Fish Kill hotline at toll-free 1-800-636-0511 or submit an online report at MyFWC.com/FishKill.

"We have not seen anything like this in the past," agency spokeswoman Carli Segelson said in an email to the Tampa Bay Times.

Most of the turtles that have been found dead along the St. Johns' 310-mile length are Florida softshell turtles, one of the most common freshwater turtle species in the state. They are also one of the largest fresh­water turtles in Florida, with fleshy shells adapted for swimming, a long neck and an elongated head with a nose like a snorkel.

A few river cooters — another large freshwater turtle species but with a flatter appearance — also have been found dead, Wildlife Commission officials said in a news release.

Freshwater turtles are considered an indicator of the health of the entire ecosystem — a reptilian version of canaries in a coal mine but with shells instead of feathers.

Just this week, the environmental group Public Employees for Environmental Responsibility filed complaints with the U.S. Environmental Protection Agency about the level of pollution in the St. Johns from five municipal sewer plants along the river, plus the Georgia-Pacific paper mill.

All six are discharging above the amount of pollution allowed in their state permits, PEER contends.

"Florida allows the St. Johns River to be treated like an open sewer," Florida PEER director Jerry Phillips, a former state pollution-enforcement attorney who drafted the complaints, said in a news release about the EPA complaints.

Ten years ago, rising demand for turtles in China for food and medicine led to the roundup of thousands of freshwater turtles from Florida's lakes, ponds and canals.

The Wildlife Commission then imposed stiff limits on the harvest of wild turtles.

Contact Craig Pittman at craig@tampabay.com. Follow @craigtimes.