While measures have been taken to make it much more difficult for terrorists to hijack passenger jets, and use them as weapons, there are many aircraft that don't fly with a dozens of passengers ready to jump on any hijacking attempt. Yes, there is still another source of large aircraft that terrorists can use for suicide attacks. There are two thousand air freighters in service, all of them pretty big and many freighter versions of the 747. Some 500 of these belong to FedEx and UPS. Some two thirds of them operate out of the United States. These aircraft fly with few people on board; a crew of two or three plus, in some cases, a few passengers. These aircraft operate from most major airports, using their own freight terminals. While security has been increased at the freight operations, false IDs, and perhaps bribes, could be used to get a hijacking team into the area. The terrorists could have one or two people get jobs in a freight terminal for a few months in order to collect information the hijacking team would need. Finding out how to smuggle the team onto a freighter, and then take over the aircraft in flight, is more complicated than the September 11, 2001 operation, but it's doable. 

But there are even easier ways to do it. There are smaller air freight operations in Eastern Europe that are open to any reasonable offer. These are the guys that run guns and other illegal items anywhere. For a price. What the terrorists could do here is lease one of these aircraft, have it fly to the U.S. and, well, you can figure out the rest. This also brings up another angle. While the United States has increased it's airport security quite a bit, such is not the case for many foreign airports. So the hijackers might find it easier to snatch a freighter from, say, Africa, bring it across the Atlantic and make for the nearest nuclear reactor or high rise building.