The man who went on a shooting rampage in Northern California this week found an easy way around a court order prohibiting him from having guns: He secretly built them at home.

Police say Kevin Neal made the two high-powered rifles he had when he was fatally shot by police Tuesday after he killed his wife and four others.

It's the latest case of homemade weapons being used in a crime and it comes as federal authorities try to draw attention to what are called "ghost guns." They can be easily built from kits purchased legally over the internet and contain no registration numbers, making them untraceable.

While making a ghost gun is legal, selling one is not. Federal officials are trying to crack down on illegal mills where such guns are built and sold.