More than 25,000 bees were found dead in Wilsonville, Ore. (The Xerces Society for Invertebrate Conservatio/AP Photo)

The mystery of why thousands of bees fell from the sky has been solved, according to the Oregon Department of Agriculture.

The department announced Friday that it has determined an insecticide caused the deaths this week of 25,000 bees in Wilsonville, Ore.

The bees were found scattered across a parking lot earlier this week.

Mace Vaughn and his partner Rich Hatfield of the non-profit environmental group the Xerces Society worked with the Oregon Department of Agriculture to discover the cause by painstakingly picking up specimens of dead bees.

"We've lost a hundred, a hundred fifty colonies at least just from this area - just wiped them out," Vaughn told ABCNews.com affiliate KATU-TV in Portland.

On Friday, the Oregon Department of Agriculture determined the bees were killed by an insecticide called Safari used to kill aphids. The trees where the insecticide was used are being netted to protect any surviving bees that might wander into the area.

The death of the bees in Oregon comes as colony collapse disorder threatens honey bee populations across the U.S.

According to the U.S. Department of Agriculture, beekeepers have been reported losing between 30 to 90 percent of their colonies since 2006. There is no known cause for the disorder, in which bees abruptly leave the hive.