Tires: Tire Forensics, performance tires, tire sidewall

Question
About 4 months ago bought a pair of Douglass Performance tires and installed them on the front.  Everything was working fine until I left work one day and discovered that the inside of the tire had an oval cut about 3" in length, and 1.5" width. toward the inside rim.  I took it to the place where I had bought it, one said there was a 'clean' cut and somebody must have cut it.  The other one in the shop said it was a blow out.  Can you help me determine this?  

Answer
This sounds like a hole that was made by an object slicing into the sidewall.

If you were to look carefully at the sidewall shape at the part of the tire that is in contact with the ground, you will notice that if you were to pass a vertical plane through the sidewall parallel to the long axis of the car (the direction of travel), the shape described by the intersection of the plane and the tire sidewall would result in an oval.

So the conclusion must be that something "sliced" off the sidewall, even though it is in an unlikely location.