Yes. You can get a pistol kit that will work on an AR-15 type lower and you can also get a Thompson Center Contender (single shot pistol) in .223 (or practically any other caliber for that matter).
As for the police, I sure hope they aren't just looking for white vans and calling it at that. There is obviously no assurance they are restricted to the same vehicle, two days into it. Physical descriptions of the suspects from video camera evidence, if there is any as some reports claim, would obviously be more useful. And should be released to the public if available.
Some may also be wondering what can be done about such tactics. The obvious answer is simply for the guns all sorts of ordinary people have to be out and waiting for the perps. The easiest way to stop this kind of thing is for an armed bystander who happens to be on the spot to intervene. If you think the 1 out of 250 in the police forces can do it all for you, think again. They can investigate afterwards, but not protect everywhere. Something like 1 out of 5 civilians armed, can.