WADR, you are citing cases where there had been multiple occurances all within the same general area. In those cases, it makes sense to warn the public, because there is clear evidence the public as a whole is not safe. What the OP was asking for, were cases where a crime was committed, and LE came out and struck fear in the public by stating there is a killer on the loose? In a case like this, where they didn't have any evidence that this was a serial case, I think LE generally relies on the public to use common sense. In all fairness to them, they also stated that parents should be aware of what their children are doing.
I'm sure, God forbid, if a similar crime happened near the area in the future, and LE had evidence it was linked to this case, they would undoubtedly warn the public, "There is a killer on the loose!" JMO