Large munitions are prohibited from being carried in checked or carry-on luggage. They were seized Monday evening while the teens were transferring to a flight to Seattle.
The TSA contacted the FBI and Chicago police who responded and evacuated the baggage room until the items could be cleared.
The bags belonged to a 16-year-old and a 17-year-old who were returning from a school field trip to Europe. The teens were questioned then allowed to travel onward. They weren't charged
The teens told law enforcement they obtained the shells at a French World War I artillery range. It was not clear how.
TSA spokesman Jim McKinney says a bomb disposal crew determined the shells were inert and no one was ever in danger.TSA explosives experts believe they are French 77 mm shells.
Small arms ammunition, including under .75 caliber and shotgun shells, can be packed in checked baggage in accordance with airline policies. Passengers who bring prohibited items to the airport are subject to possible criminal charges from law enforcement and civil penalties from TSA.
The Associated Press contributed to this story.