WILLIAM BRANGHAM: New and troubling questions are emerging about what caused last month's Air India crash in Ahmedabad, which killed 260 people.

A preliminary investigation, released this weekend ruled out mechanical or maintenance issues.

The 15-page report found that the Boeing 78 Dreamliner's fuel cutoff switches, which stop fuel from flowing to the engines, had both been flipped, in sequence, just seconds after the plane took off, and not long before it crashed.

Experts believe it's most likely that one of the pilots was responsible.

For more on the latest in this report, we're joined by our aviation correspondent, Miles O'Brien.

Miles, so good to see you.

Let's talk about those fuel switches and how they seem to have been shut off right after takeoff.

How do they work?

Is it possible that someone bumps it with their elbow and switches it off?

Is it possible there's some mechanical issue with those switches?

What do we know about that?

MILES O'BRIEN: Those switches, William, are designed to be readily available to the flight crew in the event that they need to cut off the fuel because of, say, an engine fire.

But because they need to be readily available and they're so crucial, they are guarded in two ways.

They are spring-loaded.

You have to pull up on the switch.

And then when, you put it in place, it latches in place, so it's actually secured in two separate fashions.

Its, dare I say, impossible to just brush aside it and move that switch.

It's not something that could be done casually, and it points very strongly to deliberate intent.

In addition to that, William, there was no evidence of any sort of mechanical problem with the engines, whether the engines might have failed in some fashion and there was an incorrect procedure to restart the engines.

That was one of the early speculations, but there's zero evidence that occurred.

The flight data recorder simply records the fact that those switches were absolutely, inexplicably turned off.

WILLIAM BRANGHAM: There was this in the report a mention of a very odd conversation between the two pilots about these switches and who turned them off or what happened.

What did the investigators find out?

MILES O'BRIEN: The flight data recorder, we don't know who is talking to whom at this stage of the game.

They haven't released that, but one of the pilots says to the other: "Why did you just do that?"

And the response was: "I didn't do that."

So there was a recognition that something very drastically wrong happened and on the other end of the equation, a denial.

So, again, that points toward some sort of deliberate action and, for some reason, no recognition of the fact of what just happened.

WILLIAM BRANGHAM: Given that there is still a lot that we don't know, but let's for the sake of conversation assume that this was an intentional act, is it -- I'm correct that this is not the first time that this has happened with a flight?

MILES O'BRIEN: William, it isn't.

I have taken a look back.

And going back to 1982, that's the first case I can find, it occurred with the Japan Airlines flight in Tokyo.

There have been nine separate occurrences that could be attributed to the flight crew taking some sort of deliberate action, in other words, suicide and homicide.

Now, I am including the missing Malaysian Airliner MH370, and that's a controversial statement there, but most of the thinking right now is that that is what happened there.

And that also includes this case in Air India.

So nine separate incidents, more than 1,100 people dead throughout all of those incidents.

So you couldn't say this is a common thing, but it's not something that is unprecedented either.

It's a rare occurrence, not something that I think people should worry about too terribly, but to the extent that it is very difficult to get at ways to solve it, it's something that the aviation community is grappling with.

WILLIAM BRANGHAM: I mean, obviously mental health is spread all the way across society, but, as you're indicating, when it comes to pilots, by nature of their job, they have so many people's lives in their hands.

What is the screening mental health wise for pilots?

And do the people that you have been talking with believe that it is enough?

MILES O'BRIEN: Well, it's interesting, William, because the pilot who's flying your airliner has a physical exam every six months, and - - but there is no specific psychological evaluation.

The doctors say, hey, how are you feeling?

You depressed?

And they say, I'm fine.

The problem is that there's a trap here for pilots.

If they self-report a problem that they're depressed or whatever, it's likely they will be grounded.

And this gets right to their livelihood.

And so there is no incentive for them to self-report if they're dealing with mental health issues.

So it's a real trap, and the system has to sort of rethink how it approaches this, I think, and be more proactive in ensuring that pilots are not grappling with mental health issues.

Some of the things we could look at is a more formalized approach to dealing with mental health issues, making those psychological exams more part of the routine.

Another thing I would advocate for are video recorders on the flight deck so we can see more of what's going on.

Any time you get into coming up with systems to prevent this, though, you might put a flight crew into a situation where a system inhibits their ability to respond for an emergency.

So, at the end of the day, we have to trust the individuals who are at the pointy end of these airliners.

And that leads us to kind of the inscrutable area of the human mind.

And it's -- for aviation people, it's more about psychology, whereas we're more accustomed to dealing with changing procedures.

WILLIAM BRANGHAM: Miles O'Brien, always great to talk to you.

Thank you very much.

MILES O'BRIEN: You're welcome, William.