In 1997, as a means for raising revenue, the city and School District of Philadelphia sold a portfolio of approximately 30,000 real estate tax liens worth $75.5 million to the Philadelphia Authority for Industrial Development, who then sold them to Wachovia Bank, N.A. as trustee for the bond insurer. U.S. Bank, N.A. as trustee became the successor in interest to Wachovia, empowering them to recover the delinquent taxes by foreclosing on the encumbered properties to bring them to sheriff sale. These privately owned real estate tax liens are for unpaid tax years prior to 1997, some dating back to the 1970s. Because real estate tax delinquencies continued to accrue after 1996, the city of Philadelphia also holds liens on these properties, second in priority to the older U.S. Bank liens. Although the majority of U.S. Bank lien-encumbered properties are vacant lots, hundreds of them are in use by low-income residents who are rapidly losing them at sheriff sales each month.