Alva Review-Courier -

Most mass shooters in the U.S. acquired the weapons they used legally because there was nothing in their backgrounds to disqualify them, according to James Alan Fox, a criminologist with Northeastern University who has studied mass shootings for decades.

But in several attacks in recent years gunmen acquired weapons as a result of mistakes, lack of follow-through or gaps in federal and state law.

Not all gun purchases are subject to a federal background check system. Even for those that are, federal law stipulates a limited number of reasons why a person would be prohibited from purchasing or...