With a stunning, $6 billion bid for Great Western Financial Corp., H.F. Ahmanson & Co. raised the curtain on what industry observers say could be a prolonged takeover drama of uncertain outcome.

Put in play by an unsolicited offer that escalated in value by 11% during Tuesday's stock-price run-up, Great Western is expected to attract at least one other suitor to a contest reminiscent of Wells Fargo & Co.'s $11 billion acquisition last year of First Interstate Bancorp.

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