Northwestern Mutual Life, a leading life insurance provider, agreed Monday to buy Frank Russell Co., a family-owned investment management firm in Tacoma, Wash.

Terms were not disclosed.

Thomas Courtney Jr., an investment banker with Courtney Group Inc., New York, estimated the deal's worth at $1 billion to $1.2 billion.

The deal, expected to close within six months, would give Northwestern access to Frank Russell's institutional clients.

Northwestern also plans to expand its life insurance business by selling some of Russell's proprietary mutual funds exclusively with Northwestern's variable life insurance products, a Northwestern spokesman said.

Northwestern has no plans to start selling through banks, the spokesman added.

Frank Russell, known for its family of market indexes including the Russell 2000, would keep its name and management team. It has $42 billion of assets under management and more than $1 trillion of assets under administration.

The company is 60% held by its chairman, George F. Russell Jr., and 40% owned by its 1,400 employees. Northwestern has assets of more than $76 billion.

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