NICK SCHIFRIN: Welcome to the "News Hour."

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 Sy.

STEPHANIE SY: Tears that won't stop, pain that's inconsolable.

At a memorial set up in the Muan International Airport, devastated families paid their respects to loved ones.

Five employees from this office were among those killed.

Lee Kwi-Sun mourns the loss of a colleague.

LEE KWI-SUN, Colleague of Plane Crash Victim (through translator): 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.

STEPHANIE SY: Investigators continue to gather evidence on what caused the crash.

MILES O'BRIEN: 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?

STEPHANIE SY: "News Hour" science and aviation correspondent Miles O'Brien, who is a licensed pilot, said the crew appeared forced to make split-second decisions.

MILES O'BRIEN: 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.

STEPHANIE SY: 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 Jung-Hyun.

KANG JUNG-HYUN, Senior Official, South Korean Ministry of Land, Infrastructure and Transport (through translator): If there had been more time, they could have requested preparation measures, such as dispatching fire trucks to stand by and preparing for an emergency landing.

STEPHANIE SY: The runway design itself may have proved fatal.

There's only one runway at that airport, and there was a concrete barrier if one overshot the runway, which is exactly what happened.

MILES O'BRIEN: Yes, it's not what anybody would consider a smart or safe airport design.

The antennas that sit on top of that reinforced concrete berm at that airport have to be where they're located because they're designed to keep the airplane centered on the center line of the runway.

In this case, it was way overengineered.

It was reinforced concrete and a mound, which sat right in the middle of what we call the overrun area of the runway.

And if it weren't for that, you can't help but wonder how many people might have survived.

STEPHANIE SY: Muan Airport's own internal guidelines released earlier this spring had flagged that the airport failed to comply with the recommended length space at the end of the runway, and a reinforced concrete berm used to House navigation antennas was too close to the runway.

MILES O'BRIEN: Every accident is something where the aviation community learns a little more about safety.

And this is one piece that might have been overlooked, unfortunately, which, hopefully, this accident will lead to similar obstacles not being in place at runways all over the world.

STEPHANIE SY: The investigation is far from over.

Officials say retrieval of data from the cockpit voice recorder has begun, as has the work of identifying the remains of the 179 victims.

For the "PBS News Hour," I'm Stephanie Sy.

NICK SCHIFRIN: The disaster comes against the backdrop of increasing political uncertainty.

Today, a court in Seoul issued an arrest warrant for impeached President Yoon Suk Yeol, part of an investigation into whether his declaration of martial law earlier this month amounted to rebellion.

Yoon was impeached by South Korea's opposition-led Parliament on December 14, sending one of the United States' most important allies into political freefall.

The acting president has been on the job for just five days and has declared a weeklong period of mourning following the crash.