BOISE, Idaho (AP) - Officials still don't know why as many as 2,500 mallard ducks have died in a bizarre cluster along a southeastern Idaho creek bed, but preliminary test results indicate a bacterial or fungal infection could be to blame, a state game official said late Thursday.

More tests are now planned on water and grain, said David Parrish, supervisor for the Magic Valley region of the Idaho Department of Fish and Game.

"We have some preliminary results," he told The Associated Press in a telephone interview. "It could be some type of bacterial infection or a fungal-related infection. But we haven't confirmed that for sure."

Parrish and members of the Idaho Department of Environmental Quality, the state Department of Agriculture, the federal Homeland Security Department and U.S. Fish and Wildlife Service conferred by conference call late Thursday.

He declined to say specifically what was discussed, but didn't rule out that more ducks might die in the area.

"We may have a few more, but that's a little difficult to predict right now until we can determine the exact cause of the mortality," he said.

He said the die-off was not typical.

"It's fairly uncommon, especially in these types of numbers and in such a confined area," he said.

The tests were being performed at the Fish and Wildlife Service's national laboratory in Wisconsin, the University of Idaho and Washington State University.

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, Parrish said. He said 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 earlier that 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.

Drew said the ducks likely were exposed to a single contamination source and gathered at the creek, their mutual roosting point, to die.

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

The ducks might have contracted a bacterial or fungal infection by eating grain treated with pesticides by local cattle farmers, Drew said earlier. Farming chemicals might 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. 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.

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