For more, I'm joined by Alan Diehl, a former investigator for the National Transportation Safety Board, and the author of "Air Safety Investigators: Using Science to Save Lives."Alan Diehl, I want to walk you through some of the theories we have heard so far today and which we heard just now in that tape. Let's talk first about the complete midair collision or failure. What is the possibility of that?

　　ALAN DIEHL, Former NTSB Investigator: Well, the wreckage — the wreckage location pattern suggests that there was some kind of inside breakup, but I don't think this happened at 31,000 feet, even though it spread over several miles.

　　That's a fairly tight pattern, and there's been report that the data was streaming as low as — down to as low as 5,000 feet. So the problem may have began at 31,000 feet, but I believe the aircraft was relatively intact down to lower altitudes, perhaps as low as 5,000 feet.

　　Now, we do know the tail is three miles from the main wreckage and there is other debris and bodies scattered in the area, but I don't think this aircraft came apart, like, for example, the Malaysia 17 flight did when it was hit by a large missile over Ukraine.

　　GWEN IFILL: So it's not consistent with a midair explosion, at least not at that height?

　　ALAN DIEHL: I would say not, Gwen.

　　But, of course, having done this 40 years, everything is very preliminary and I can only speculate. I think the Egyptians will do a very thorough job. We know that, oftentimes, people say this is like putting a jigsaw puzzle together. Only, many times, you don't have all the pieces.

　　I think we have got all the pieces right now. The recorders were in good shape. The Egyptians had analyzed those. I think that was why they discounted the missile strike theory. And, of course, the wreckage is in a desert area. This is not like the Ukrainian crash, where it was scattered over a very wide area. People were pilfering things, we think, and maybe even the rebels were removing things.

　　So I think we have got all the parts of this jigsaw puzzle, but it's going to take a little time for all the players to come together. It's not just the Russians that are doing the investigation. We're using the Egyptians. We know there are five countries.

　　GWEN IFILL: Right.

　　ALAN DIEHL: And particularly with the French involved, I think we're going to get to the bottom of this, and fairly expeditiously.

　　GWEN IFILL: You mentioned the fact that the plane's tail was some distance away from a lot of the other wreckage.

　　There's also — there were also reports today that there had been damage to the plane's tail some years ago that had been repaired. What's the potential for some sort of weakness or metal stress and failure which would have created something this catastrophic?

　　ALAN DIEHL: Well, we have seen this before. In a 747 over Japan, the worst single aircraft accident in the history, killed 520 people, the tail came off, because a — they're called tail strikes. The aircraft tail bumps the ground on takeoff or landing.

　　So this is a real possibility. But, again, Gwen, they have the physical evidence, and the Egyptians seem to be doing a very good job of protecting that. I have noticed they have even put fences around the primary wreckage location.

　　So, the metallurgists will haul that tail back into the lab and, with electron microscopes and other techniques, establish whether this repair was done correctly or whether there was some sort of metal fatigue involved or even a catastrophic failure for some other reason. I think we will get to the bottom of this when we have the evidence.

　　GWEN IFILL: Well, the black box, is that essential for us to find out whether maybe pilot error could have been involved?

　　ALAN DIEHL: Absolutely.

　　And, of course, as many people have said, everything is still on the table. Another thing that was very curious about this, and, again, these are initial reports confirmed I guess by the Egyptians, that the radar data suggested that the plane didn't just suddenly dive out of the sky from 31,000 feet, that it actually entered a series of undulations where it climbed and dived — dove a couple thousand feet before it made its final plunge toward the desert.

　　So, that could — we have seen this in the past with these highly automated aircraft, where pilots have been confused and perhaps shut down the wrong system. The automation is fed by other sensors. And sometimes these sensors, like in the Air France accident in the South Atlantic, they — in essence, they're feeding erroneous data to the computers and the computers do things that cause the pilots to be confused and contribute to this.

　　Now, you can argue that's pilot error or design error. Take your pick. The lawyers will sort all that out in future years, I guess.

　　GWEN IFILL: Alan Diehl, aviation security consultant, thank you for helping us.

　　ALAN DIEHL: Thank you, Gwen.