WASHINGTON -- An internal Homeland Security Department memo obtained by WorldNetDaily advises airports and air carriers to tighten security over passenger aircraft and air cargo during the holiday season.

U.S. officials explain that air cargo and the back side of airports – known as the ramp, where jets are serviced – are still relatively vulnerable to terrorism more than two years after al-Qaida terrorists hijacked and crashed four jumbo jets on the East Coast.

Based on recent al-Qaida chatter, U.S. intelligence remains concerned that the terrorist group plans to use jets to attack nuclear, chemical or hazardous-materials facilities during the holiday season.

"Al-Qaida could attack U.S. LNG [liquid natural gas], nuclear and chemical facilities (manufacturing and HAZMAT storage) using aircraft, either passenger or cargo aircraft, the latter loaded with explosives," warns the Homeland Security memo, which was distributed Nov. 21 to federal agencies and law enforcement.

"Additionally," the document states, "we cannot discount multiple attacks involving the use of general aviation aircraft."

The 5-page internal advisory, marked "For Official Use Only," recommends aviation security officials beef up security beyond existing security directives and emergency amendments issued by the Transportation Security Administration. Existing directives focus on the passenger side of the airport, where pre-gate security screening is done.

Specifically, the memo advises officials to tighten ramp, or airside, security, where the catering, cleaning, fueling and maintenance of aircraft takes place.

"Secure unattended aircraft to prevent unauthorized use," the memo recommends as one of several additional protective measures. The Homeland Security Department requested WorldNetDaily not publish the other measures.

Despite concerns expressed in the memo, which was not publicly released, the department has not raised the terror threat alert for the public. It remains at yellow.

Aviation security experts have complained that the ramp is still open to terrorism. Planes that remain overnight are "not guarded, and in fact, frequently the door is open," said Charles G. Slepian, an airline security analyst in New York.

"So if you have access to the ramp, you have access to the plane," he said. "You just walk up the stairs and you walk into the plane. And if you want to plant a bomb, you can. And you leave the same way and nobody is the wiser."

Slepian and others also worry that planes are rarely inspected for explosives and weapons. And ramp workers are screened for them at only two of the nation's 428 commercial airports.

Homeland Security press secretary Brian Roehrkasse says that without congressional authority, the department can only offer the private sector guidelines on tightening ramp security. He says Congress mandated that aviation security focus on passenger screening in the wake of the 9-11 hijackings.

"But as you see in that document," he told WorldNetDaily, "we have given some fairly specific stuff that they can use as part of designing their security plans."

The document also outlines an eight-point plan for tightening air cargo security. Again, the government has asked WorldNetDaily not to publish the protective measures.

Unlike checked passenger luggage, air cargo is not screened for explosives.

"Air cargo is not being inspected, and that is a big concern," Slepian said. "It is an extremely vulnerable area."

Roehrkasse explains that Congress mandated that only luggage be screened for bombs.

"There was no direction given in terms of air cargo," Roehrkasse said in a phone interview. "There are no explosive detection systems for cargo."

He adds that such a mandatory system might be cost-prohibitive. The giant explosive-detection machines recently installed in airports, while the size of minivans, still aren't big enough to handle large cargo.

"We spent several billion dollars just putting in the EDS systems for the checked passenger luggage, and you've seen them in the airports – they're not that big that you could have a giant parcel go through them," Roehrkasse said. "And the cost of creating even larger machines that are the size of a house at all cargo airports to have these things go through would obviously be upwards of billions and billions and billions of dollars."

As WorldNetDaily first reported earlier this week, the Homeland Security memo also warns U.S. law enforcement to prepare for possible al-Qaida car-bombings and cyanide-gas poisoning of subways during the holiday season. Fox News Channel and NBC News have confirmed the exclusive reports.

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