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.
Keltec 9 (FP-9 and the -11 both have long creepy trigger pulls. Double action only with striker.
If you fire (as many as you want) and leave one in the chamber - then insert a new - and full magazine, you will the situation reported. Toss the ‘used’ mag into the bushes, trash, whatever.
Easy to to do if the cops aren’t looking real hard.