"At large..." is use to describe a situation where a person or persons remain free. I.e. not yet caught by the authorities. in the context of this article, it meant to say there is no other shooter besides the one they've already caught.

At large here means they don't believe there is another shooter. Often during these events the first reports are there are more than one shooter. Later almost every time they find there was only one. So they are saying there is no other shooter loose.

Thank you for all , who helped me.