According to the story on CNN.com, the guy claims he thought he was shooting at a squirrel. They also say that, after saying he was going to get help, he instead went to a birthday party, went home to play video games, and then went to work.
Thanx for the additional info. Saturday was a drizzley, rainy overcast day in central Wisconsin--i.e., visual conditions were not the best. But that's not an excuse--only more reason to be doubley sure of the target and what is beyond.