First Interstate Bancorp sued Wells Fargo & Co. earlier this week, claiming in documents filed in Delaware federal court that the San Francisco bank, which is making a hostile bid for the Los Angeles company, had "embarked upon a campaign of deceit and manipulation designed to undermine the merger (with First Bank System Inc.) and force the First Interstate board to accept Wells' offer."

First Bank System, which has reached a friendly agreement to buy First Interstate, filed a similar lawsuit last week in Delaware, where all three companies are incorporated.

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