What is the name of your state (only U.S. law)? Minnesota

We purchased our first home in September, 2008. The seller purchased this home as a foreclosure and 'flipped' it after some remodeling. It quickly became apparent that there is no way a licensed plumber worked on this place before we bought it. The seller added a basement bathroom with stool, sink, and shower. We have major issues with sewage backups in first floor facilites when basement bathroom is used. We also believe that it is possible that whomever did the work on this place may have tied a storm drain in the basement in with a sewage line or vice versa. My question is would we have some recourse with the seller if he authorized this kind of thing and did not disclose this to us? At one point I believe that it is 'hand written' into our purchase agreement that property is sold 'as is'. Does this obsolve him of any disclosures? Please help!