SoftBank Group agreed to buy asset manager Fortress Investment Group LLC for $3.3 billion, in a surprising move that is part of an effort by the Japanese technology giant to transform itself into one of the world’s largest investment firms.

Fortress Class A stockholders are to get $8.08 a share, the companies announced late Tuesday after The Wall Street Journal reported on the deal. That is 39% above the closing price Monday, excluding dividends. The shares surged by about 6% Tuesday before word of the acquisition surfaced.

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