If one of the murderers did have a gun, then I apologise.
But you are unable to read news reports pausing so I'm going to give it one more go and then I'm going to put the dinner on.
The sentence you quote from that Telegraph article - Police fired again at the hand holding the gun and shot Adebowale’s thumb off - is reported speech. It's not a statement. It's the Telegraph reporter's summary of the evidence.
You can tell that because it's not in quotation marks - though in your post you put them round it.
We don't know what the police intended to do.
I guess they just wanted to stop him and that it wasn't a Hollywood thing where Clint Eastwood drills someone and then blows smoke from his magnum.