(CNN) -- The thousands of birds that fell from the sky just before midnight New Year's Eve in Arkansas likely died from massive trauma, according to a preliminary report released Monday.
The birds, most of which were dead when they were found, were red-winged blackbirds and starlings. They were found within a one-mile area of Beebe, about 40 miles northeast of Little Rock, the Arkansas Game and Fish Commission said.
Keith Stephens, a spokesman for the 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.
He cited a preliminary report conducted by the Arkansas Livestock and Poultry Commission.
"Further tests will be done to rule out other causes, but the birds suffered from acute physical trauma leading to internal hemorrhage and death. There was no sign of any chronic or infectious disease," the report said, according to the game and fish commission.
As of Saturday, between 4,000 and 5,000 birds had been found dead, said Stephens.
Karen Rowe, an ornithologist for the game and fish commission, said the incident is not that unusual and is often caused by a lightning strike or high-altitude hail.
A strong storm system moved through the state earlier in the day Friday. Officials also speculated that fireworks shot by New Year's revelers in the area might have caused severe stress in the birds.
Blackbirds do not normally fly at night, and it was not immediately clear what caused the odd behavior. Loud noises were reported shortly before the birds began falling, according to the game and fish commission.
"The birds obviously hit something very hard and had hemorrhages," said Rowe.
"Initial examinations of a few of the dead birds showed trauma. Whether or not this trauma was from the force of hitting the ground when they fell or from something that contacted them in the air, we don't know," she said.
In a seemingly separate incident, some 500 red-winged blackbirds, starlings and grackles were found dead in southern Louisiana in Labarre.
The birds showed no evident trauma, according to Jim LaCour, a veterinarian with the Louisiana Department of Wildlife and Fisheries. Samples have been sent to Wisconsin and Georgia for testing, he said.