according to this article.

Six airline flights between Paris and Los Angeles were canceled at the U.S. Embassy's urging because of terrorism fears, the French government and Air France said Wednesday.

The flights scheduled for Wednesday and Thursday were called off because of information obtained "in the framework of the French-American fight against terrorism," the French prime minister's office said.

The U.S. Department of Homeland Security had been meeting with French officials in recent days over concerns about a possible terrorist attack.

At the end of a long flight like that, the planes would be fairly low on fuel. While still able to inflict a lot of damage, they might not be nearly as dangerous as the planes hijacked on 9-11. Because they'd have much less fuel on board, they'd also have a lot less mass, therefore a lot less kinetic energy as well as less explosive power. Still, they could kill a lot of people if crashed into a crowd.