Packages intended to be placed on a truck, like the bomb that exploded Tuesday at a FedEx facility in Texas, are not screened as carefully as items carried by passenger planes.

Largely that is because of the high cost of screening every parcel intended for domestic delivery.

Delivery companies such as FedEx and UPS rely on a risk-based strategy. They hope to detect illegal or dangerous shipments by spotting something unusual about the package or the shipper. Some security experts give the companies good marks while pointing out the limitations of their approach.

FedEx and UPS say only that they have security measures in place and cooperate with law enforcement. They declined to discuss specifics, saying that would compromise security.

Here are some questions and answers about security of parcels:

ARE ALL PACKAGES SCREENED?

Cargo on passenger planes must be screened, usually by computed-tomography scanners although explosive-trace detection and dogs are also used, said Jeffrey Price, an aviation-security expert at Metropolitan State University in Denver.

If a package is going to be placed on a truck for delivery within the United States, as with the device that exploded on a conveyer belt at a FedEx facility in Schertz, Texas, "there is much less likelihood that it's going to be physically screened with X-ray or even a person examining the package," said John Cohen, a former counterterrorism coordinator at the Department of Homeland Security.

HOW ARE SHIPMENTS CHECKED?

For truck shipments, cargo carriers train employees to look for suspicious behavior, including anything that looks odd about the package, or a shipper who buys too much insurance for what he says is in the box, Cohen said. Those procedures developed in the 1980s to detect shipments of drugs or guns and evolved to be used to find explosives.

An employee at a FedEx center in Austin, Bryan Jaimes, 19, told reporters he never received new guidance from managers about handling packages as Austin authorities look for what they've called a "serial bomber." He said his job is to load the trucks and that he assumes other workers earlier in the shipping chain give packages a once-over before they get to him.

FedEx and UPS officials declined to say whether they screen ground-shipping packages at drop-off points or distribution centers. On Tuesday, investigators closed off an Austin-area FedEx store where they suspect that the bomb was dropped off.

The most stringent screening rules apply to packages that will be carried on passenger airplanes.

FedEx and UPS each have their own fleet of planes, and the rules are not as strict. Price said the companies aren't required to use X-ray, explosive-trace detection or canine screening but can at their option. He said they are required to physically inspect all packages.

HAVE TERRORISTS TRIED TO HIDE BOMBS IN CARGO?

Yes. The threat posed by bombs given to delivery companies was highlighted in a 2010 plot aimed at blowing up planes flying to the United States.

Bombs hidden in printer cartridges were shipped from Yemen but intercepted in Dubai and the United Kingdom and were defused. The bombs were pulled off U.S.-bound planes after officials got a tip from authorities in Saudi Arabia. The U.S. then banned large toner and ink cartridges from passenger planes and ordered new inspections of high-risk shipments on cargo planes coming into the country.

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David Koenig can be reached at http://twitter.com/airlinewriter