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IT’S YET ANOTHER HORRIFYING, “what if” post-9-11 scenario. Nearly 100,000 fans pack the Rose Bowl in Pasadena, California, to watch the year’s national college football championship game unfold when, suddenly, multiple small explosions erupt in one section of seats, followed by successive explosions in other seats.

Dozens are killed and hundreds injured, all on live national television. It’s a public spectacle to rival 9 11—a terrorist’s dream. Speculation centers on suicide bombers, but the cause is even more unnerving: GPS-guided, grenade-dispersing mortars, fired from the hills six miles away, by terrorists who fled discreetly by car before the rounds even reached their target.

The scenario comes not from a movie or an expert peddling a book, but from respected think tank RAND Corp., in a report requested by the U.S. Department of Homeland Security (DHS) Science and Technology Directorate, about the prospect of advanced conventional weapons falling into terrorists’ hands.

The report considers technologies including computerized sniper targeting systems, antitank weapons, and new, “smart” exploding firearm ammunition, which is deadly even if the user does not have a clear line-of-sight to a target. Each, RAND says, is a “game changing” weapon that could subvert common security practice because “attacks could come from far beyond any controllable security perimeter.”

Terrorists have only sought to obtain one, better-known advanced conventional weapon: man-portable air defense systems (MANPADS), more commonly known as shoulder-fired anti-aircraft missiles, according to the report. They have been used against civilian airliners on several occasions, including a failed 2002 attack on an Israeli airliner taking off from Mombasa, Kenya.

RAND, however, found that U.S. and international efforts to combat the spread and use of MANPADS offer a model for preventing the spread and use of other such conventional weapons.

The best method for preventing the weapons’ use, RAND argues, is inclusion of technical controls, such as a “trusted component” necessary for the system to work. For example, existing technology could require encryption keys for projectiles, like mortars, to use guidance data. Such controls, however, are usually only effective when built into systems from the design phase, not added as an afterthought. But with regard to advanced conventional weapons, codes or keys like those required to use nuclear weapons are viewed by the U.S. military as an unwanted inconvenience that could prove deadly in the field of battle.

There have been efforts to restrain trafficking in these weapons. In 2005, the United States convicted British arms dealer Hemant Lakhani of attempting to sell Russian MANPADS to a cooperating U.S. witness. The witness had told Lakhani that he wanted to purchase the weapons so that he could shoot down a U.S. airliner.

The expected growth in use of one specific system—GPS-guided mortars—and terrorists’ historical use of conventional mortars, in places like Iraq and even Northern Ireland, leads RAND’s experts to recommend U.S. action, and “soon.” This should begin with technical study by DHS and other federal agencies, and diplomatic collaboration with foreign countries. Even if these controls are not airtight, they would ideally make it too difficult, expensive, and risky for terrorist groups to want to acquire the systems, says the study.

In the field, awareness provides the best defense against advanced conventional weapons that have already fallen into the wrong hands, RAND’s experts argue. That means that protective agencies like the U.S. Secret Service and its counterparts, as well as operators of critical infrastructure sites, must now consider unseen threats from as far as a mile away, not just those within physical view.