(CNN) -- Arkansas game officials hope testing scheduled to begin Monday will solve the mystery of why up to 5,000 birds fell from the sky just before midnight New Year's Eve.

The birds -- most of which were dead -- were red-winged blackbirds and starlings, and they were found within a one-mile area of Beebe, about 40 miles northeast of Little Rock, the Arkansas Game and Fish Commission said. Birds fell over about a one-mile area, the commission said in a statement.

As of Saturday, between 4,000 and 5,000 birds had been found dead, said Keith Stephens with the commission.

"Shortly after I arrived, there were still birds falling from the sky," said commission wildlife officer Robby King in the statement. He said he collected about 65 dead birds.

The commission said it flew over the area to gauge the scope of the event, and no birds were found outside of the initial one-mile area.

Karen Rowe, an ornithologist for the 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.

"It's important to understand that a sick bird can't fly. So whatever happened to these birds happened very quickly," Rowe told CNN Radio on Sunday.

"Something must have caused these birds to flush out of the trees at night, where they're normally just roosting and staying in the treetops ... and then something got them out of the air and caused their death and then they fell to earth," Rowe added.

Officials also speculated that fireworks shot by New Year's revelers in the area might have caused severe stress in the birds. Rowe said Sunday there was evidence that large fireworks may have played a role.

"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," Rowe said.

The dead birds will be sent for testing to labs at the Arkansas Livestock and Poultry Commission and the National Wildlife Health Center in Wisconsin.

The necropsies will begin Monday, Stephens said, and the findings should be available sometime this week.

The city of Beebe has hired U.S. Environmental Services to begin the cleanup and dispose of the dead birds, the commission said. The firm's workers will go door-to-door and pick up birds still in yards and on rooftops.

CNN's Tina Burnside and Shelby Lin Erdman contributed to this report.