SEOUL (AP) — The first report on last month’s Jeju Air crash in South Korea confirmed bird strikes in the plane's engines, though officials haven’t determined the cause of the accident that killed all but two of the 181 people on board.

The preliminary accident report released Monday said feathers and bird blood stains were found in both engines.

“The samples were sent to specialized organizations for DNA analysis, and a domestic organization identified them as belonging to Baikal Teals,” the report said, referring to a migratory duck.

The report also said the plane's black box stopped recording about 4 minutes before the crash.

South Korea earlier announced that it will remove a concrete structure at the end of the airport's runway that was involved in the crash.

Some experts have said that Muan International Airport’s localizer — a set of antennas in a concrete structure that guide aircraft during landings — likely made the crash of the Jeju Air plane worse.

The Boeing 737-800 skidded off the airport’s runaway on Dec. 29 after its landing gear failed to deploy, slamming into the concrete structure and bursting into flames. Many observers said the structure should have been made with lighter materials that could break more easily upon impact.

Investigators have said that air traffic controllers warned the pilot about possible bird strikes two minutes before the aircraft issued a distress signal confirming that a bird strike had occurred, after which the pilot attempted an emergency landing.