Welcome to the NewsHour. American investigators have arrived in South Korea to help investigate the year's worst airline crash of a Boeing 737-800 south of Seoul.

But today we learn the airport's own internal guidelines questioned the placement of a barrier that likely made the crash far more deadly and has led to an outpouring of grief for the families of the

179 people who died. Here's Stephanie Sott. Tears that won't stop, pain that's inconsolable.

At a memorial set up in the Muon International Airportastated families paid their respects to loved ones.

Five employees from this office were among those killed.

Lee Kui's son mourns the loss of a colleague.

I still can't believe it's real until now.

She was promoted recently and all of a sudden this happened. I still can't believe it.

I miss her smile.

Investigators continue to gather evidence on what caused the crash.

I think the thing that has stumped a lot of aviation experts is why that aircraft came in with its wheels up and its flaps not deployed. Obviously, before landing,

you like to have both of those, the flaps down and the gear down. So the question was,

why would the crew elect to do this? NewsHour Science and Aviation correspondent Miles

O'Brien, who was a licensed pilot, said the crew appeared forced to make split-second decisions.

This was a crew that didn't just lose one engine, but might have lost both for some reason, either through birds or some other problem, and put them in a situation where they wanted to get back on that runway as quickly as possible.

Video by an eyewitness on the ground shows the plane landing on its belly,

skidding off the runway and slamming into a concrete embankment.

On Sunday, the South Korean Transport Ministry said the pilot aborted an initial landing

after air traffic control issued a bird strike warning.

He then made another approach, but within a couple of minutes issued a Mayday call.

It all happened too fast, said Transport Ministry official Kang Zhang Hewn.

If there had been more time, they could have requested preparation measures, such as dispatching

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