(CNN) -- Some of the nation's top experts Tuesday were looking for clues into sudden, mass deaths of birds in two states over New Year's weekend.

"This one is unusual because of the time period over which so many birds died," said LeAnn White, a field investigator with the National Wildlife Health Center in Madison, Wisconsin. The center, part of the U.S. Geological Survey, is receiving samples from Arkansas, where as many as 5,000 red-winged blackbirds and starlings fell from the sky in a square-mile area in less than an hour on New Year's Eve, according to the Arkansas Fish and Game Commission.

The center will also examine samples from Louisiana, where 500 red-winged blackbirds, starlings and grackles were found dead in Labarre.

A preliminary report conducted Monday by the Arkansas Livestock and Poultry Commission found that the birds in Beebe, Arkansas, likely died from massive trauma.

There had been reports of loud sounds in Beebe before the reports of birds falling began to come in. It's possible the sounds made the birds disoriented, and they went into sudden, chaotic flights, crashing into each other and into objects, White said. "You're disturbed, you're disoriented, you're trying to figure out where you are. We have seen some stuff like this before when there's heavy dense fog, and they'll run into towers and power lines," she said.

Other bird experts agree that that's a likely explanation. Still, the sudden deaths are quite unusual. "It's kind of a freak event," said Greg Butcher, director of bird conservation for the National Audubon Society. "You just don't see these kinds of mass deaths very frequently at all."

At this time of year, blackbirds are in huge roosts, particularly throughout the southeastern United States, he said. They generally don't fly at night.

"In the course of a year, about half the birds that are around die -- even under the best circumstances. The life of a bird is brutish and short. But it just usually happens out of the sight of people," said Butcher.

It's possible that the loud sound that triggered the incident was New Year's Eve fireworks, though officials said the investigation is ongoing.

"If it had been any other day of the week," Butcher said, he might have suspected people to be behind the deaths. "There's a lot of purposeful killings. ... People really don't like these big roosts. But I can't see anybody doing that on New Year's Eve."

Doug Inkley, senior scientist with the National Wildlife Federation, said his "first suspicion is that they were frightened and went into a mass panic."

"I would call this event unusual," he said, "but not unprecedented."

Keith Stephens, a spokesman for the Arkansas Game and Fish Commission, said the birds showed evidence of trauma in the breast tissue, with blood clots in the body cavity and a lot of internal bleeding. All major organs were normal.

Karen Rowe, an ornithologist for the commission, said such incidents can be caused by a lightning strike or high-altitude hail. A strong storm system moved through the state earlier in the day Friday.

The experts CNN spoke with saw nothing surprising in there being a similar report of bird deaths in Louisiana. "It's another one of those coincidences," said White, adding, "I'm really interested to see if there's anything that could possibly be related. I asked about weather conditions there. There's sometimes high-altitude hail." But, she said, so far officials were not aware of "any strange weather patterns" in the area.

Butcher, with the National Audubon Society, said there may be more reports of dead birds found around the country. "As soon as one of these things hits the airwaves, people are just more sensitive to seeing dead birds around."

Meanwhile, Arkansas officials are investigating the death of an estimated 100,000 fish in the state's northwest, but suspect disease was to blame, a state spokesman said Sunday.