According to the CNN article and the ABCNews article, on their respective websites, the Pentagon's official word at present is that the US is not sure whether the damage was the result of an "errant bomb" that the US dropped as part of a response to anti-aircraft fire over the region, or whether the anti-aircraft fire itself – falling back to earth – was responsible.

There's a question as to whether there was a deliberate anti-aircraft attack, or whether the US pilots mistook the firing of guns as part of the wedding party to be a military attack. But even if these people were shooting as part of a celebration, it's a dangerous practice (those bullets fall back to earth too, y'know), especially when your town is being flown over by an opposing military force with bombs! How many brain cells does it take to know better? Sheesh!!!

The four injured people in question are civilians that US forces themselves had airlifted to a hospital in Kandahar, so they're the only casualties the US can be sure of. The other casualty figures, at this point, are speculation until confirmed.

Here's the ABCNews article: http://abcnews.go.com/sections/world/DailyNews/STRIKE_MAIN.html