"The property was given to the town in December by a bank that acquired it from the Lanza family. Since the shooting more than two years ago, the house stood vacant."
This means that after the boy and his mother were deceased, the home was either foreclosed upon by the bank (pretty likely in these circumstances) or the estate quit-claimed it to the bank in lieu of a pending foreclosure. The bank bought goodwill in the community by giving it to the town. IIRC, the boy and his mother lived there and the father of the shooter (divorced from the mother) lived somewhere distant. If the divorce deeded the property to the mother, and she refinanced, or used it as collateral for loans, or simply had a lot of debts in her estate, there may have been negative equity in the house, or no net equity in the estate. The executor or administrator of the mother's estate may have just let it go into foreclosure, or actively given it back to the bank to save time.