The story is breaking. More details as they are available.
From McGwire's official statement:
"I'm grateful to the Cardinals for bringing me back to baseball. I want to say thank you to Cardinals owner Mr. DeWitt, to my GM, John Mozeliak, and to my manager, Tony La Russa. I can't wait to put the uniform on again and to be back on the field in front of the great fans in Saint Louis. I've always appreciated their support and I intend to earn it again, this time as hitting coach. I'm going to pour myself into this job and do everything I can to help the Cardinals hitters become the best players for years to come.
"After all this time, I want to come clean. I was not in a position to do that five years ago in my Congressional testimony, but now I feel an obligation to discuss this and to answer questions about it. I'll do that, and then I just want to help my team."