Two South Boston office buildings owned by Normandy Real Estate Partners have been transferred to special servicing because of an imminent mortgage default, the ratings agency Fitch Ratings said today.

Normandy acquired the two Fort Point buildings, at 281 and 321 Summer St., in a market-topping transaction. In 2007, Normandy paid the Archon Group $73 million, or nearly $285 per-square-foot, for the two properties.

Normandy financed the $73 million purchase with a $51 million mortgage from UBS. UBS then packaged the mortgage into a pool of commercial debt and sold it to Wall Street investors. Normandy also financed the purchase with a $15.2 million mezzanine loan that UBS originated and then sold to New York investment REIT Capital Trust. Capital Trust will act as the loan’s special servicer, Fitch said.

The loan has been on a servicer’s watch list since September 2008 because of vacancy issues, according to the mortgage debt-tracking firm Trepp.

Nearly 75,000 square feet of the two-building portfolio’s 257,000 square feet sit vacant, according to LoopNet listings. The building at 281 Summer St. has suffered some high-profile vacancies lately, with the architecture firm Cubellis and the restaurant and clothing boutique The Achilles Project both folding.

Last year, Normandy swooped in and grabbed Boston’s John Hancock Tower at a discount when that trophy asset’s over-leveraged owner, Broadway Partners, defaulted on a debt payment. Normandy’s troubles in Fort Point show how even buyers who are finding opportunities in the current round of commercial distress are also being haunted by properties they bought at the market’s peak.

The commercial mortgage-backed securities (CMBS) delinquency rate hit 7.6 percent in March, according to Trepp. More than $47.8 billion in CMBS debt was delinquent as of March, according to the data firm RealPoint.

Justin Krebs of Normandy’s Boston office could not be reached for comment.