When you restore the power in the building, go down to the "Digging Site". When you get there, you will see a case with something in it. Make sure you have plenty of Timed Mines. Walk towards it and throw four or five Timed Mines at it then run away. When it blows up, the woman on the speaker will start talking. People with very good guns (Flame Thrower, Shotgun, Machine Guns, etc.) will appear. You will also have a new objective to not let any mutants survive.