BOISE, Idaho (AP) - The number of mallard ducks that have died in a bizarre cluster along a southeastern Idaho creek bed has climbed to 2,500, as vexed wildlife officials await test results from tissue samples.

Officials from the Idaho Department of Fish and Game and the U.S. Department of Homeland Security hope Thursday to have results of tests on tissue from the ducks' abdominal tract and on water samples from the creek that might provide clues to the unknown illness.

The battery of tests at the U.S. Fish and Wildlife Service's national laboratory in Wisconsin, the University of Idaho and Washington State University are expected to rule out an avian flu outbreak.

The ducks mysteriously began dying last week around Land Springs Creek, near the remote town of Oakley, about 180 miles southeast of Boise. On Thursday, state workers cleared the last remaining duck carcasses from the area in pickup trucks. They brought the bodies to a nearby incineration site.

Migratory mallards from Canada and their local cousins staggered and struggled to breathe before collapsing, said Dave Parrish, regional supervisor for Fish and Game. He said Thursday every mallard in a radius of several miles has died - approximately 2,500, up from an earlier estimate of 1,000.

"I've never seen anything like this in 20 years here," he said. "There were dead mallards everywhere - in the water and on the banks. It was odd; they were in a very small area."

The massive outbreak is puzzling scientists because only mallard ducks are dying. Golden eagles, geese, magpies, crows and other birds in the area all remain healthy.

In the past, small outbreaks of botulism have killed water birds in Idaho, but the disease quickly spreads among different species.

"Typically, you'd see this spread into other types of waterfowl as well," Parrish said.

Mark Drew, a wildlife veterinarian with the state Department of Agriculture, said investigators were not ruling out any cause of death, but bird flu virus remained unlikely.

The symptoms - bacterial lesions in the lungs and hemorrhaging in the heart wall - probably point to a bacterial infection, he said.

"The highly pathogenic bird flu that we see in Southeast Asia has not been reported in North America," he said. "It would be extremely unusual that this would be the first occurrence of avian influenza."

Drew said the ducks likely were exposed to a single contamination source and gathered at the creek, their mutual roosting point, to die. He did not suspect the mallards were passing a contagious virus.

No dead ducks have been spotted anywhere besides the creek. Investigators did not find any dead marine life in the shallow stream.

The ducks may have contracted a bacterial or fungal infection by eating grain treated with pesticides by local cattle farmers, Drew said. Farming chemicals may also have spilled into the small spring-fed creek, which measures just 3- to 6-inches deep.

Farmland surrounds the backwoods waterway. A cattle feedlot is close by and several corn and alfalfa feeds ring the nearby town of Oakley. Parrish said there are no factories in the area that discharge toxins into local streams and rivers. Wastewater does not run into the creek, he said.

Along with Idaho Fish and Game and Homeland Security officers, representatives from the U.S. Department of Agriculture, the Idaho Department of Environmental Quality and the local health district were investigating the deaths.

The agencies posted signs warning hunters not to eat any birds killed near the creek.

"I'd say there's no reason for alarm in the sense that literally the sky is falling and there's disease spreading," Drew said. "It's unusual in the number of birds and the sense that it's only mallards, but it's nothing that would cause anyone to panic."