Transcript for 4 taken into custody after Australia plane threat

Matt dowd, thanks very much. To those chilling details on those terror arrests in Australia. Four men taken into custody, reportedly planning to release a Leddy toxic gas to take down a passenger plane. Security at aijports stepping up. James Longman is tracking the latest from London. Good morning, James. Reporter: Good morning, Amy. It looks like this could have been a devastating terrorist attack, and unlike so many of the more improvised random killings we've seen in recent months, this time there was a plan. This morning airports across Australia on high alert after a plot to take down a plane is foiled. There has been a major joint counter-terrorism operation to disrupt a terrorist plot to bring down an airplane. Reporter: Five Sydney homes arrested. Two fathers and two sons of Lebanese descent yet to be charged. We believe it's islamic inspired terrorism. Reporter: Details scarce but one local newspaper with details. Toxic gas to kill or immobilize everyone on board. Sources telling ABC news American intelligence officials are closely monitoring the case. Homeland security says they are assisting Australian investigators and have taken extraordinary steps to protect flights bound for the united States. All this comes just days after the TSA announced new security measures across U.S. Airports citing an increased threat to airport security. Any electronic item bigger than a cell phone will need to come out of your bag at the screening area. George. Thanks. Let's get more from our military analyst Stephen ganyard. We know that Al Qaeda and ISIS have been focused on planes as a top target for more than a decade but this is a different kind of a threat. How would it work? That's a good question, George. It's actually unique. We've never seen this before. Apparently the Australian police seized something like a meat mincer or grinder about the size of a bread loaf. So you would think is this a binary weapon, two chemicals that would get mixed together, is it sort of a gas canister? Hard to say. It would have to be something really big to affect the whole airplane. Remember the air conditioning recirculates the air every three minutes. The pilots could dump the cabin pressure or dump the oxygen masks. It's certainly chilling that ISIS and Al Qaeda continue to be obsessed. It shows insight into their thinking while we're focused on some kind of an explosive set off in that way, they're moving in a different direction. Exactly. It's sort of this technological tit for tat. We know that they're trying to get different kinds of laptop bombs that can get by our screening machines so they're trying something else here. They're going to continue to work and find innovative ways to bring down airplanes or affect the general flying public. Is this kind of threat something you had heard about before or is this brand new? This is brand new. I looked back and I couldn't find anything that has to do with gas. It just shows you how determined they are to bring down an airplane and get around screening procedures. Like you say, it's chilling. Steve ganyard, thank you very much.

This transcript has been automatically generated and may not be 100% accurate.