Because weapons can be easily carried across state lines, states that try to impose tight controls on gun purchases can be undercut by other states that are more permissive, analysts say.

“There’s no metal detectors at the borders,” said Northeastern University criminologist James Alan Fox. “Any state’s ability to deal with the gun problem is limited by that fact: They’re not isolated islands.”

Gun-control advocates say that’s one reason Congress needs to strengthen federal background checks of prospective gun buyers, limit the size of magazine clips and ban military-style “assault” weapons.