AMNA NAWAZ: Welcome to the "News Hour."

Investigators are trying to determine what led to one of the worst airline accidents in decades.

More than 240 people died today after a London-bound Air India flight crashed just moments after takeoff.

GEOFF BENNETT: All but one of the 242 people on board reportedly perished.

And authorities are trying to determine how many other people were killed on the ground when the plane crashed into a building in the city of Ahmedabad in Western India.

As John Yang reports, it was the first fatal accident with a Boeing 787 Dreamliner.

A warning: This report contains graphic video.

JOHN YANG: It all happened in less than a minute.

Air India Flight 171 took off at 1.39 p.m. local time bound for London, on board, 242 passengers and crew.

It climbed to just 625 feet and then started sinking.

Within moments, the Boeing 787 Dreamliner crashed just outside the airport perimeter, hitting a hostel for medical students.

Videos from the site show the plane's tail jutting out from the building.

RAJMAL MENARIA, Victim's Relative (through translator): My daughter's father-in-law, his name is Vardi Chand, was on the flight.

I dropped him to the airport and came home, when we got the message that the plane had crashed.

JOHN YANG: Another video shows the plane's slow descent as if it was landing.

Indian officials said the pilot issued a mayday call, the international distress signal, but the flight crew didn't respond to subsequent calls from air traffic controllers.

Within seconds, a fireball and thick black smoke.

Air India CEO Campbell Wilson.

CAMPBELL WILSON, CEO, Air India: I know that there are many questions.

And, at this stage, I will not be able to answer all of them.

The investigations will take time, but anything we can do now, we are doing.

JOHN YANG: Miraculously, passenger Ramesh Viswash Kumar walked out alive, the sole survivor.

He was assigned seat 11A next to one of the main exit doors.

It's said he jumped off the plane.

Of the 241 others on board who perished, there were 169 Indians, 52 British, seven Portuguese, and one Canadian, among them, 22-year-old Nganthoi Sharma, a flight attendant, her family in Manipur in Northeast India, inconsolable.

Dr. Komi Vyas and her family of five posed for this smiling selfie on board the ill-fated plane.

The family was planning to move to London.

MAN: Goodbye, India.

MAN: Goodbye.

MAN: Thank you.

MAN: And our flight back to England.

JOHN YANG: Two British nationals, Jamie Ray Greenlaw-Meek and Fiongal Greenlaw-Meek, posted this video shortly before boarding.

And former Gujarat -- visit his family.

Some of the dead were on the ground.

The plane crashed as residents of this hostel were sitting down to lunch.

Some of the medical students were killed, their meals half-eaten, their bright futures extinguished.

For the "PBS News Hour," I'm John Yang.

GEOFF BENNETT: The plane's black box has been recovered, which could provide some answers about what transpired before the crash.

Investigators from the U.K. and the U.S. will travel to India to work with a team on the ground there.

For more on all this, we're joined now by our aviation correspondent, Miles O'Brien.

So, Miles, having seen the video of the plane taking off, what stood out to you?

MILES O'BRIEN: Geoff, it seemed like a normal takeoff roll.

They used quite a bit of the runway, but it was a very hot day there, and so the air molecules were less dense, and that would be expected.

It climbed out as you might expect,.

And then at the point you would imagine the landing gear being stowed away to continue the climb, things stopped moving and progressing.

The ascent settled down, leveled off, and then slowly but surely it went into the ground, apparently an aerodynamic stall.

So either this aircraft lost thrust and could not continue its climb or it lost lift.

And one of the things that strikes me is why the landing gear was still down as it went down.

It should have been stowed by that point.

And if you look at the trailing edge of the wing and look at where those flaps are, those aerodynamic surfaces which are extended during the takeoff and landing phases of flight, slower portions of flight to provide more lift, does not appear they were deployed.

So, some sort of misconfiguration here, perhaps, some sort of loss of thrust or maybe some combination of that, Geoff.

GEOFF BENNETT: Might those questions be answered by the black box which was recovered?

MILES O'BRIEN: I think almost 100 percent likely, Geoff, that the black box, both of them will shed light and give us an idea of what happened here.

There are literally thousands of streams of telemetry captured by the solid state black box on a Boeing 787, advanced airliner that it is.

And the cockpit voice for its quarter itself, which begins when the crew powers on the aircraft at the gate, will indicate if there was some miscommunication about whether the flaps or landing gear should have been stowed or if there was some misunderstanding about the configuration of the plane or the power settings.

There's any number of things that could have led to this, but I'm pretty certain we're going to get an answer because of the recovery of those boxes.

GEOFF BENNETT: And, Miles, there's been scrutiny in the past over the design of the 787 Dreamliner.

There's no indication the design played a role in this incident, but how does that history inform what the investigators will do when they get on the ground there?

MILES O'BRIEN: Well, it's important to remember that the investigation, a good one, never puts blinders on.

All factors are considered, and by process of elimination, you get down to the cause of an accident.

Boeing, in the early days of the 787, they had problems with the batteries, had to ground this fleet for quite a while, but it's had a sterling record ever since, no crashes, no fatalities.

We have heard recently from whistle-blowers that there are some questions raised about how the composite fuselage was joined together and whether it was shimmed properly.

No reason to indicate that that had anything to do with this particular event.

Having said all of that, investigators will be looking at those complicated systems that are meant to keep the aircraft safe to make sure it was doing its job as well, Geoff.

GEOFF BENNETT: Aviation correspondent Miles O'Brien.

Miles, thank you.

MILES O'BRIEN: You're welcome.