Is it just a blank t-shirt, or does it have printing on it.
Is it just the letters BYC on the label of the t-shirt? Any other information there? If so, there might be an RN number (I think it's a 9 digit number) at the bottom of the label.
If you look up the RN number at the
FTC Website, it should tell you the manufacturer.
It could be that BYC removed the original manufacturers label and put in their own, but if they are the manufacturer, the RN number will tell you.