Terrorists who are trying to exploit cargo planes to launch an attack on the U.S. may find a security weakness in screening of cargo planes flying over, though not into, the United States, but security experts say targeting overflights is a waste of scarce resources.

The new focus on overflights was raised after the Transportation Security Administration confirmed to The Washington Post in Monday editions that planes that go over this country but aren't supposed to land here are not routinely screened according to U.S. standards.

TSA confirmed to the newspaper that other countries "have their own cargo security protocols that apply to those aircraft," and the new Secure Flight program in the U.S. to scrutinize passengers boarding overflights is not yet under way. 

U.S. homeland security officials say they use a risk-based approach to air transportation, which means they can't protect everything all of the time and have to pick their marks. Overflights are by necessity lower down the list.

Steve Emerson, executive director of The Investigative Project on Terrorism, said he agrees with the approach.

"Everything is possibly a gamble in terms of being potentially exploited by terrorists, but terrorists generally know which is going to be the more successful way of causing as much damage as possible. Using those overfly flights is not going to cause as much damage," he told Fox News.

A former TSA intelligence official told Fox News that part of the reason behind the lesser concern is that the "vast majority" of overflights originate in Canada, and the Canadians know how to screen. It is "not some Third World country," according to the official.

Homeland Security Secretary Janet Napolitano said Sunday that public locations on the ground are of greater concern to security officials trying to dismantle threats. 

"We look at so-called soft targets, the hotels, shopping malls, for example, all of which we have reached out to in the past year and a fair amount of training for their own employees, as well," Napolitano said on CNN's "State of the Union."

U.S. officials say terrorist networks are trying to exploit cargo planes because it is so much harder to get operatives onto U.S. flights with weapons or explosives. That in itself is seen as an indicator that other measures are working.

But after this fall's failed printer-bomb attempt by Al Qaeda in Yemen, cargo screening itself has been beefed up. In the Yemeni case, the bomb being carried aboard a cargo plane bound for the United States contained an explosive, PETN, which is non-metallic and cannot be picked up by X-ray. As a result, dogs and trace detention are being used more widely on inbound cargo planes.

"I would say even a greater threat is the threat of private commercial aviation," Emerson said,  "because no one is really checking those flights for private commercial passengers and nobody is checking to make sure that the cargo that gets loaded onto those flights doesn't contain explosives."

Current air security rules for cargo are dictated by the law passed in the wake of the Sept. 11, 2001, attacks on the United States, which was based on recommendations of the Sept. 11 Commission. A TSA official pointed out that the law does not include cargo on overflights, though other screening procedures are in place.  

The law does provide for 100 percent of cargo carried aboard domestic flights to be screened, as is 100 percent of cargo carried on passenger flights leaving the United States. 

But on passenger flights coming into the United States from overseas, there is still a gap. A TSA official told Fox News on Monday that only two-thirds of that cargo is screened.

Napolitano said her "fail rate" goal for contraband getting aboard a plane is "zero."

"I mean we want nothing to get aboard a plane that is not safe," she said.

Emerson said it's neither practical nor possible to "protect every single vehicular or transportation hub." He suggested trains are also a greater risk and can cause more damage, "which is why we've seen attempts to blow up or plots to blow up commuter trains in different cities."