The mysterious deaths of thousands of migratory birds at a military firing range has been traced to a chemical released into a marsh where the birds feed, the Army said Friday.

"For more than 10 years, the cause of catastrophic waterfowl mortality in Eagle River Flats has remained a mystery," scientists said in a report that appears to solve the enigma.A team of scientists from the Army's Cold Regions Research and Engineering Laboratory in Hanover, N.H., and Dartmouth Medical School concluded that white phosporous, used to produce smoke in high-explosive artillery shells, has been poisoning ducks and swans that stop off to rest and feed during spring and fall migrations.

Army spokesman Chuck Canterbury estimated that 2,000 ducks and swans have been killed annually but said no one knows exactly how many birds have died on the 2,500-acre salt marsh north of Anchorage used as a firing range by Fort Richardson soldiers.

No birds were killed by artillery fire or by explosions. They dropped dead after feeding in sediments of shallow ponds.

Live-fire exercises were halted last year during the $1 million investigation into the bird deaths. The U.S. Fish and Wildlife Service has urged the Army to permanently close the artillery range and make it a conservation area for the birds.