Last week’s close call involving explosive packages addressed to U.S. targets brought fresh scrutiny to a post-9/11 mandate to screen 100 percent of cargo shipped within, into, or out of the United States on passenger planes—a mandate remains unmet, months after its August 2010 deadline.

The packages from Yemen, which were discovered with the help of intelligence relayed by Saudi Arabia and not as a result of screening, have sparked calls for better cargo screening not only for the cargo carried on passenger planes but also for packages carried on all-cargo planes. Some transportation experts, however, aren’t convinced that screening every package is a sensible solution.

Here’s what we wrote in 2008 about the 9/11 cargo screening mandate, which applies only to passenger planes:

Under the 9/11 Commission Act of 2007, the [Transportation Security Administration] is required to screen 50 percent of cargo on passenger planes by February and 100 percent by August 2010. In October, the TSA announced that it had met the 50 percent deadline early by focusing on smaller airports and on regional jets which make up more than 90 percent of passenger planes.

Getting to 100 percent is more complicated. Because regional jets have narrow bodies, they mostly carry piece cargo as opposed to the giant pallets of bulk containers hauled by planes with two aisles in the cabin. Those containers have to be broken down, screened and reassembled.

A subsequent report by the Government Accountability Office, released just about a month before the August 2010 deadline, noted that “it is questionable, based on reported screening rates, whether 100 percent of such cargo will be screened by August 2010 without impeding the flow of commerce.”

While the TSA faced challenges relating to domestic cargo as well, packages inbound from other countries posed a bigger problem. “TSA does not expect to achieve 100 percent screening of inbound air cargo by the mandated August 2010 deadline,” according to the report, which noted that the agency had admitted as much in Congressional hearings months earlier:

In a March 4, 2010, hearing before the Subcommittee on Homeland Security, House Committee on Appropriations, in responding to questions, the Acting TSA Administrator stated that it could take several years before 100 percent of inbound cargo is screened. According to TSA, screening inbound air cargo poses unique challenges, related, in part, to TSA’s limited ability to regulate foreign entities.

Most of TSA’s security efforts have focused on the screening of cargo carried on passenger planes, which typically carry some cargo in addition to passengers’ luggage. According to the GAO, this represents about 16 percent of all cargo shipped to, from, or within the United States.

The rest, which is shipped on all-cargo planes, receives no systematic screening. According to The Wall Street Journal, the percentage of shipments on all-cargo planes that is screened before touching down in the U.S. may be as low as 50 percent. USA Today noted that Rep. Ed Markey, D-Mass., fought in 2005 for stricter screening requirements for all-cargo planes—an effort that was met with “a very successful lobbying effort made by the cargo industry,” according to Markey.

Fixing the vulnerabilities by screening every piece of cargo could cost billions and still wouldn’t provide complete protection. Transportation experts and analysts told the Associated Press and The New York Times such measures may not be realistic.

Bruce Schneier, a security technologist, weighed in on the cargo bomb plots on his blog, advocating full screening for cargo on passenger planes but warning against the impulse to expand screening to all cargo.

Terrorism on cargo planes “isn’t very terrorizing. Packages aren’t people,” Schneier wrote. “If we're so scared that we have to devote resources to this kind of terrorist threat, we've well and truly lost.”

Schneier said terrorism is fought “not by defending against specific threats, but through intelligence, investigation, and emergency response."

That also appears to be the position of Homeland Security chief Janet Napolitano, who told NPR’s All Things Considered that the bomb plot showed that the country’s security system worked. She touted a “multilayered system, out of which intelligence-sharing is the first layer,” but she sidestepped a question about whether routine airport screenings would have discovered the explosives.