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Good questions in context of gun control law discussions. Haven't heard any info on that. Maybe someone else did?

Just heard: illegal weapon, serial no. unrecognizable.

And it seems: No IS relation -> amok run

So he simply bought on black market.....

He bought the gun on the dark web. It seems, somebody from Czech Republic or Slowakia reactivated a deactivated weapon certified for use as a theatrical prop.

It's not yet clear, though, where he got 300 rounds of ammunition from.