US Officials Warn of Airline Shoe Bomb Threat

For the second time in less than three weeks Homeland Security officials are warning about potential attempts to smuggle explosives onto airplanes.

"I think aviation symbolizes something to al Qaeda, that the idea that you can take down a plane and kill potentially hundreds of people -- both that are on the plane and on the ground -- is very appealing," former FBI agent Brad Garrett said.

The Department of Homeland Security said Wednesday terrorists are working on new designs for shoe bombs, liquid explosives, and explosives hidden in cosmetic containers -- all items that can be used to target flights entering the United States from overseas.

"We're going to see more activity, more screening, more random questions to individuals," terrorism expert Walid Phares said.

As security increases, travelers can expect more pat-downs and extra scrutiny of their shoes and carry-on items.

The latest warning comes as security officials acknowledge a growing threat to U.S. national security: a Qaeda's increasing presence in Syria.

U.S. officials said operatives linked to the terror network are working their way into the rebel forces there and pushing out moderates.

Syria, or parts of it, could potentially become a safe haven for new terrorist bases, training grounds for attacks against the United States.

In January, National Intelligence Director James Clapper told Congress there are an estimated 26,000 Islamic extremists in Syria. About 7,000 of them are foreigners.

The White House said it's examining its options in Syria, including drone strikes on rebel factions that might later try to attack the United States.

Critics said the terrorists wouldn't be so strong if President Barack Obama had responded sooner and more forcefully to the Syrian crisis.

"In the Mideast this situation was predicted and predictable," Sen. John McCain, R-Ariz., said. "And the stakes here frankly are incredibly high."