Conshohocken-based Keystone Property Group has paid $114 million for the office building formerly known as Penn Mutual Tower at 510-530 Walnut St. near Independence Mall, the company said in a release.

Keystone plans to convert the ground-floor and mezzanine office space of the 880,000-square-foot building - now called One Washington Square - into retail, it said.

Keystone is also an owner of the nearby Curtis and Dow Buildings. The company purchased the One Washington Square property from Rubenstein Partners of Philadelphia, which acquired the building in late 2013.

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