Many office property owners that have been able to keep their heads above water are being undone by tenant contractions and the expiration of five-year leases that were signed at the peak of the boom.

Penn Mutual Towers, an office complex across the street from Independence Hall in Philadelphia, has seen its vacancy rise and income fall after one big tenant left and another renewed its lease for 15 percent less than it had been paying. Its creditors are foreclosing on the property, according to data company Trepp LLC.

Similar problems are mushrooming in office markets throughout the country, foreshadowing a new wave of real-estate trouble.