Investigators will try to determine what happened on the plane, including how its mechanical systems were functioning and which actions the pilots took.

As South Korean investigators investigate a Jeju Air crash that killed 179 people on Sunday, experts say crews probably will focus on whether the jet struck birds, why its landing gear and flaps had not extended, and why a concrete barrier was built at the end of the runway.

The Boeing 737-800 crash-landed at Muan International Airport near the country’s southern tip Sunday morning before slamming into the barrier and bursting into a ball of fire, according to video from the ground.

Two members of the cabin crew survived the deadliest airline disaster in six years. One is able to communicate, and his account of the flight could be vital to the investigation.

Hassan Shahidi, president of the U.S.-based nonprofit Flight Safety Foundation, said investigators will want to piece together what caused the plane to crash and what made the incident so deadly.

“This is a very complicated accident, and it involved numerous factors that investigators will have to study,” Shahidi said.

Experts cautioned that limited official information has been released or emerged from witness accounts and video, and that numerous questions remain about what happened on the flight in the minutes before the fiery crash. South Korea’s Aviation and Railway Accident Investigation Board is leading the investigation, with assistance from American teams from the National Transportation Safety Board, the Federal Aviation Administration and Boeing. A preliminary report should be issued in a month, but the complete investigation probably will take well over a year.

Kristy Kiernan, associate director of the safety center at Embry-Riddle Aeronautical University, said the timeline reflects the need to chase down every lead — even those that might not initially seem significant — to help prevent similar crashes.

“It is really the bedrock of a safety program to approach it from that perspective,” Kiernan said. “Our number one goal is improving safety.”

While the concrete barrier did not cause the plane crash, experts said its presence at the end of the runway may have made the crash far deadlier. John Cox, a safety consultant, said that video showing the plane skidding down the runway suggests pilots had maintained a degree of control.

“They land on that runway beautifully,” Cox said. Had the barrier not been there, he added, the plane might have had enough space to come to a stop safely.

It is not uncommon for planes to run over the ends or sides of runways. The placement of structures nearby is governed by international standards, Shahidi said, and investigators will want to know whether those were adhered to. For example, objects near runways are supposed to be “frangible,” meaning they break apart on impact.

Airport officials said the structure was designed to keep navigation equipment level with the runway, which slopes at the end, according to the semiofficial South Korean news agency Yonhap.

Doug Moss, a retired airline pilot, said the layout of the airport appears to be a major element of the crash. Moss said it’s not unusual for runways to have a slight slope because it’s expensive to make them perfectly flat, but while he’s seen a lot of strange airport designs, “this one takes the cake.”

“You have to anticipate that someone’s going to run off the runway at some time,” he said.

Investigators will try to determine what happened on the plane, including how its mechanical systems were functioning and which actions the pilots took.

Investigators said they have recovered the plane’s black boxes, and while one suffered damage that could slow the retrieval of information, the data and flight-deck conversations they recorded will be key to understanding what happened. One of the two survivors, a 33-year-old flight attendant, is conscious and able to communicate, his doctor told reporters. His account could also provide important information on the flight’s final minutes.

Authorities have said the airport’s control tower warned the plane’s pilots of bird activity in the area before the crash, and witnesses on the ground told Yonhap that they saw the plane hitting a flock of birds and flames coming from the aircraft.

Charts of the airport note four areas of bird activity close to the runway. Authorities list countermeasures they use to keep birds away from planes, including nonlethal “gas cannons and playback of distress calls,” as well as lethal measures that include shooting birds. Shahidi, of the Flight Safety Foundation, said the investigation will examine which measures were in place at the time of the crash.

Investigators will also want to know the extent of damage birds caused to the jet. Airliners can continue flying with only one engine, but in extremely rare cases bird strikes can knock out both.

“That is a crucial part of the investigation,” Shahidi said.

The video of the crash showed the plane sliding on its belly without landing gear down or its flaps extended, which would be expected if the pilots were preparing to land. It will be up to investigators to determine whether the controls had failed or whether the crew made errors. Even if the normal systems for controlling the landing gear were not working, the jet had manual backups accessible from the flight deck.

Moss said understanding the pilots’ training and actions before the crash will be key.

“They landed too quickly,” Moss said. “They didn’t have enough time to go through any checklists.”