Is that meant to be a prediction, or a statement of fact? If you read the article it becomes clear that they had search warrants as part of a targeted investigation into organised crime, and apparently were surprised to discover the 3D printer at one of the searched areas. Given that they arrested someone because they think he was making gunpowder, and because you can't make gunpowder with a 3D printer, it seems that they believed (correctly) that someone was trying to manufacture ammo and got a judge to issue a warrant on that basis. When they discovered the printer, they made the obvious logical conclusion - someone who is illegally making guns, and has a 3D printer, might be experimenting with 3D printing plastic guns. What else would he use it for?
It may turn out in the course of events that the printer was used for something else, or making tools used to help make ammo rather than making gun parts, or something else. But ownership of the 3D printer is incidental. There isn't even any way they would know he had such a device, as far as I can tell.