It all started out innocently enough when Firstar Corp.'s executive team hosted a conference call for clients of Goldman Sachs Group to talk about how to do mergers well.

But the May 12 conference call sparked a five-day, 9% decline in Firstar's stock. The drop was accompanied by a swirl of rumors that the Milwaukee banking company was shopping for yet another acquisition, possibly even Minneapolis-based U.S. Bancorp, whose stock rose 6% in the days after the call.

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