A person can pull a gun, put his finger on the trigger, point it at someone and "unitentionally" pull the trigger. That could be called an "accident". Before I get flamed - I know the four rules and people some will say it's a negligent rather than accidental discharge. In plain English, the article implies the officer he didn't intentially shoot the man.
See definition 2a:
http://www.merriam-webster.com/dictionary/accident" onclick="window.open(this.href);return false;