HAL Real Estate Investments of Seattle sold its 7th & Madison building Tuesday for $100 million -- three times what it paid for the 9-story building two years ago.

Public records identify the buyer as an affiliate of Prudential Real Estate Investors, the real-estate arm of the insurance giant.

The sale is another sign of institutional investors’ growing interest in Seattle, and represents a big turnaround for the once-troubled downtown project, highly visible from Interstate 5.

Developer Opus Northwest completed the 205,000-square-foot office building in early 2009, but couldn’t find a tenant and eventually turned it back to construction lenders led by U.S. Bank to avoid foreclosure.

HAL bought 7th & Madison for $30.75 million in November 2010, at the same time announcing a deal to convert it to medical use and lease all the space to the Polyclinic, a multi-specialty medical clinic based on First Hill.

Tuesday’s sale is the 11th real-estate transaction in King County so far this year of $100 million or more.

Prudential, the building’s new owner, recently sold West 8th, a 28-story office tower in the Denny Triangle.

Eric Pryne: epryne@seattletimes.com or 206-464-2231