BOSTON—Rack up another major commercial office buy for Rockpoint Group, LLC. The firm has increased its portfolio here with the acquisition of the 99 Summer St. office tower here.

Rockpoint, a real estate private equity firm headquartered in Boston, acquired the property from Cornerstone Real Estate Advisors of Hartford, CT for an undisclosed sum. The deal for the 20-story, 270-000-square-foot office tower marks the third significant purchase by Rockpoint in 2015 with another major transaction in the works at press time. At the time of purchase, the property was 95% leased.

99 Summer St. has changed hands twice in nearly 10 years. In July 2013 Normandy Real Estate Partners sold 99 Summer St. to Cornerstone Real Estate Advisors for $110.8 million, or $407 per square foot. Normandy paid $68.2 million for the office tower in 2006 as part of a portfolio purchase.