In television appearances on Sunday, John O. Brennan, President Obama’s top counterterrorism adviser, said that American and British authorities were leaning toward the conclusion that the packages were meant to detonate in midair, en route to their destinations in Chicago. If that turns out to be the case, it would be a rare attack aimed at the air cargo system — one of the foundations of the global economy — rather than the passenger system, which has received the most attention from governments working to avoid a repeat of the Sept. 11, 2001, attacks.

For the most part, governments around the world had bet that it was less likely that the cargo system would be the target of attacks, given that its flights carry few passengers.

“It is time for the shipping industry and the business community to accept the reality that more needs to be done to secure cargo planes so that they cannot be turned into a delivery systems for bombs targeting our country,” Representative Ed Markey, Democrat of Massachusetts, said in a statement.

Congress in 2007, in legislation proposed by Mr. Markey, mandated that all air cargo be inspected before it is loaded onto passenger planes, setting an August 2010 deadline for the requirement. But as of the deadline, only about 65 percent of the cargo headed to the United States on passenger planes from abroad is inspected — and a far smaller proportion coming to the United States on all-cargo flights is physically checked, as these planes are not subject to the mandate.

Even when the cargo is checked, air carriers in certain countries use equipment like X-ray detection devices or a visual check by an airport worker that often cannot identify packages with bombs, because the small amount of explosive material can be carefully hidden inside a routine electronic device, like a computer printer.

Image A photo from the Saudi Ministry of Interior showed Ibrahim Hassan al-Asiri, a mail bomb suspect. Credit... Saudi Arabia Ministry of Interior, via Associated Press

Interviews in Washington, London and the Middle East reveal how the two bombs made their way through several countries before the tip from Saudi intelligence officials caused them to be pulled from airplanes.