Isn’t it possible to chamber one round then insert a full magazine? Does this particular 9mm have a hair trigger? Could have been an accident but it sounds like he was in ‘hunter’s mode’.
Yep, that's how many carry. However, when you fire the round in the chamber, the slide is always going to pull one out of the magazine to replace it. The only way to fire a semi-automatic pistol and have a full magazine is to chamber a round, and hold the magazine in your opposite hand as you fire.
It was a Kel-Tec so the slide may have over ran the top one in the magazine without moving it. I have no personal experience but have heard they were not reliable.
One thing which does bother me a bit was the neighborhood watch had phoned 911. That leads me to think the kid was doing something to merit that much concern.
It is possible, even a common practice, to chamber a round, then insert another round into the magazine to have the pistol loaded to full capacity.
When the first round would be fired, the pistol, if operating properly, would still take another round from the magazine and place it in the chamber, thus leaving the magazine with one round less than full capacity.
As this story was from an unnamed source, I think it likely that the reporter or the source simply garbled it a bit.
The useful evidence seems to be that they only found one casing at the scene. Even that does not mean much.
Legally, you are justified in continuing to shoot until you are no longer under a deadly threat.