THERE MAY NEVER be a better spokesman for closing the gun show loophole than Adam Yahiye Gadahn.

A 32-year-old American and convert to Islam, Mr. Gadahn referred to the terrorist attacks of Sept. 11, 2001, as “blessed raids” and called for additional “jihad against America.” He has been on the FBI’s Most Wanted Terrorists list and faces criminal charges for treason and for providing material support to al-Qaeda. Mr. Gadahn is believed to have left his native Orange County, Calif., for Pakistan, where he has become a multimedia front man for the terrorist organization.

His latest message, released on the Internet last week, calls on extremists in the West to take up arms against the United States.

“Let’s take America as an example,” he says. “America is absolutely awash with easily obtainable firearms. You can go down to a gun show at the local convention center and come away with a fully automatic assault rifle, without a background check, and most likely without having to show an identification card. So what are you waiting for?”

Mr. Gadahn is wrong in one respect: Individuals cannot buy “fully automatic assault” rifles without government authorization. But his larger point is well taken: Purchasing deadly weapons in the United States is easy — especially at a gun show. Buyers can scoop up semiautomatic weapons and extended magazines from private dealers at these shows without undergoing a background check.

Moreover, individuals on the U.S. terrorism watch list are forbidden from air travel but may purchase a weapon. A recent report by the Government Accountability Office documented this “terror gap”: From February 2004 through February 2010, 1,228 individuals on the watch list underwent background checks to purchase firearms or explosives; 1,119, or 91 percent, of these transactions were approved. In an update published in April, the GAO reported that some 200 individuals on the watch list were approved for weapons purchases from March 2010 through December 2010.

Mr. Gadahn’s call to arms should serve as a catalyst for the White House and Congress to move on legislation to close the gun show loophole and the terror gap. As Mr. Gadahn might say, “what are you waiting for?”