To prevent future terrorist attacks, industry and government officials are increasingly focused on the vulnerability of cargo planes -- potential fuel-packed weapons that lack some of the basic defenses now protecting passenger aircraft.

The Transportation Security Administration (search) said Tuesday that after the terror-threat level was raised to "high," it boosted the number of inspectors dispatched nationwide to ensure that companies comply with tighter cargo-screening guidelines enacted last month.

An agency spokesman said extra law enforcement now patrolling the perimeters of airports was specifically put in place to fortify air cargo facilities.

An official with Mexico's Civil Aviation Office said his government has also stepped up inspections and security at air freight terminals.

The warning from the Department of Homeland Security (searchon Sunday about a possible Al Qaeda attack this holiday season was focused more heavily on the threat from foreign commercial airliners, according to U.S. officials. But cargo planes were also cited as a special concern.

Security experts and pilots said Tuesday that the cargo industry's surveillance of airports, planes and freight -- and of the warehouse employees packing boxes -- remains dangerously inadequate, particularly among small- and mid-sized companies.

They also noted that the U.S. government has even less control over cargo and passenger planes originating in other countries, and that the most lax standards domestically generally exist at smaller airports.

The fact that only a small amount of freight carried by passenger planes ever passes through X-ray screening machines is a minor problem, these experts said, when compared with the general state of affairs among designated air cargo carriers.

Despite the advances commercial aviation has made in the realm of security over the past two years, Rafi Ron, president of New Age Security Solutions and the former security director for the Israeli Airport Authority, said "an air cargo aircraft remains just as good a weapon -- no, even better -- than passenger aircraft."

Very few cargo planes are equipped with reinforced cockpit doors, he said. Also, there are no flight attendants, passengers or air marshals who might help defend against an attack, Ron said.

Congress recently passed a bill allowing cargo pilots to carry guns, but that has not alleviated all of pilots' concerns.

James Shilling, a full-time pilot for a major cargo carrier and consultant to the Coalition of Airline Pilots Association (search), said he would like to see the TSA screen and conduct background checks on every person that has direct access to cargo planes. He also advocates screening all cargo, but says the technology must be affordable and efficient.

Stricter government standards requiring expensive equipment might force smaller carriers out of business, he said.

Indeed, a major obstacle to bolstering air cargo security is the government's fear of mandating procedures that would slow down the shipping business, if not bring an end to the next-day delivery business, a critical component of the economy.

"Everyone is concerned that there is a balanced approach," FedEx Corp. spokeswoman Sally Davenport said. Some large shippers have developed security standards in the wake of the Sept. 11, 2001, attacks that go beyond what's required by the TSA, she said.

Davenport said the Memphis-based company has intensified the screening of its employees, improved surveillance of its planes at airports and strengthened some cockpit doors.

Aside from the small percentage of air freight that is physically inspected via X-ray or other methods, the TSA considers the remainder of the nation's air cargo to be "screened" through a so-called "known shipper" program.

The program requires air cargo companies to register with the government, and it says passenger air carriers aren't allowed to accept cargo from companies that aren't on a TSA-approved list.

Some experts said it is unrealistic to consider physically inspecting every piece of freight that passes through the system.