The first thing you're going to have to do is determine exactly where the leak originates. This is going to require you to remove more drywall. I'd do it from the laundry room side of the wall. You know the pipe is dry in the attic but water is pooling around the top of the tee and running down into the basement. There is your area of concern. Remove the drywall, find the source and you're nearly there. From looking at the setup, my first guess is that a screw was put into the pipe and over the past 3 years has rusted out. However, without opening things up and getting your hands and eyes on the source, its hard to tell.