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An earlier three-judge panel in the 6th Circuit, which hears appeals for cases in Ohio, Michigan, Kentucky and Tennessee, ruled similarly in 2007 before Warshak’s trial, saying investigators overstepped constitutional bounds. But the full court the next year rejected his constitutional claim on procedural grounds in a 9-5 vote.

Dempsey said it may be years before a Supreme Court ruling decisively defines the rules on digital communications, leaving businesspeople, consumers and government officials uncertain about an increasingly important issue.

The Tuesday ruling noted an “explosion” of such communication: “Lovers exchange sweet nothings, and businessmen swap ambitious plans, all at the click of the mouse button.”