A preliminary report—released Monday by the Arkansas Livestock and Poultry Commission—suggests the birds died of trauma of unknown cause.
"The birds obviously hit something very hard and had hemorrhages," game-and-fish commission bird expert Karen Rowe told CNN.
It's unknown whether the trauma occurred during contact with something in the sky—such as lightning or high-altitude hail—or when the birds hit the ground, Rowe said.
Commission scientists began to examine the carcasses today, and final results on the exact cause of the birds' death could be back within a week.
Photograph by Stephen B. Thornton, Arkansas Democrat-Gazette/AP
Drum fish lie dead on the shores of the Arkansas River on New Year's Eve.
In addition to the dead birds, Arkansas officials are investigating what caused the deaths of between 80,000 and 100,000 fish, found Thursday by a tugboat operator near the city of Ozark, commission spokesperson Keith Stephens told msnbc.com. The cause of death will take a month to determine.
The giant fish kill occurred about 125 miles (201 kilometers) away from where the blackbirds fell from the Arkansas sky. For now officials say the two events are not connected.
Photograph courtesy Arkansas Game and Fish Commission
A worker with United States Environmental Services LLC tosses a dead blackbird off a home in Beebe, Arkansas, on January 2.
More than a thousand birds fell dead within an area about a mile (1.6 kilometers) long and 0.5 mile (0.8 kilometer) wide on New Year's Eve, commission spokesperson Keith Stephens told the Associated Press.
Such a giant bird kill has happened at least twice before in Arkansas, the AP reported: In 2001 lightning killed ducks in Hot Springs, and in 1973 hail knocked birds from the sky at Stuttgart.