DEEP IN his 1,518-page manifesto, Anders Behring Breivik, the man accused of killing 68 people at a Norwegian youth camp last month, explained how he acquired the ammunition that he used in the attack: “10 x 30 round magazines – .223 cal at 34 USD per mag. Had to buy through a smaller US supplier (who again ordered from other suppliers) as most suppliers have export limitations. . . . Total cost: 550 USD.” He says he could have purchased the clips in Sweden, but they were cheaper through the U.S. supplier.

Mr. Breivik’s claim is sadly believable, even though Norwegian officials have not confirmed the details. U.S. gun retailers can sell merchandise overseas and do not need to obtain an export license if the value of the goods being shipped falls below $100; sellers can avoid the licensing requirements — and buyers can avoid the additional costs — by breaking up the order into smaller shipments. U.S. law enforcement officials should determine whether the transaction was legal.

Several other assertions made by Mr. Breivik give credence to his claim that he obtained the high-capacity magazines from a U.S. seller: Mr. Breivik appears to have carried out the attack with a Ruger Mini 14 semiautomatic rifle — the purchase of which he chronicled in the manifesto. The Ruger uses .223-caliber ammunition – the same type meant for the magazines purchased by Mr. Breivik.

Regardless of where Mr. Breivik obtained his weapons, the events in Norway should serve as a reminder of the absurdity of producing and selling such products. No self-respecting hunter would use such exaggerated force to take down a deer. These magazines too often find their way into the hands of deranged individuals, transforming them into efficient killing machines. In Tucson earlier this year, Jared Lee Loughner relied on high-capacity magazines to tear off 31 shots in a matter of seconds, killing six people and seriously injuring 13 others, including Rep. Gabrielle Giffords (D-Ariz.). The toll would have been even greater had Mr. Loughner been able to reload his Glock 19 with a second high-capacity magazine he had on hand.

There was a brief period of sanity in this country when high-capacity magazines were prohibited as part of an assault weapons ban. That ban expired in 2004; a Washington Post review of Virginia records showed that the number of high-capacity magazines used in crimes jumped dramatically in that state after the ban lapsed. Legislation introduced by Rep. Carolyn McCarthy (D-N.Y.) to ban the sale of high-capacity magazines has been stalled since she introduced it shortly after the Tucson massacre. The terrible events in Norway ought to kick-start the measure.