Where is Micheal Phelps' father?

I have watched all of Micheal Phelps 2008 gold medal swims live and have noticed as I am sure everyone has that his mother and two sisters are always there and shown many times on camera. I was wondering where his father is during all this because it seems like something that he should be at. I know Phelps' parents are divorced but is Micheal and his father estranged or is there some other explanation. Thanks.