The initial results of an investigation into December's devastating Jeju Air crash in South Korea showed that while both of the plane's engines sustained bird strikes, its pilots turned off the less-damaged one just before its crash-landing.

The finding, which implied human errors, drew quick, vehement protests from bereaved families and fellow pilots who accuse authorities of trying to shift responsibility for the disaster to the dead pilots.

South Korea's Aviation and Railway Accident Investigation Board initially planned to publicize the results of an investigation of the plane's engines on Saturday. But it was forced to cancel its news briefing in the face of strong protests by relatives of crash victims who were informed of the findings earlier in the day, according to government officials and bereaved families.

"If they want to say their investigation was done in a reliable, independent manner, they should have come up with evidence that backs up their explanation," said Kim Yu-jin, head of an association of bereaved families. "None of us resent the pilots."

The Boeing 737-800 operated by Jeju Air landed on its belly without its land gear deployed at South Korea's southern Muan International Airport on Dec. 29. It overshot the runaway, slammed into a concrete structure and burst into flames. It was the deadliest disaster in South Korea's aviation history in decades, killing all but two of the 181 people on board.

According to a copy of an unpublished briefing report obtained by The Associated Press, a South Korean-led multilateral investigation team said it found no defects in the plane's engines, built by France's Safran and GE.

The report said thorough examinations of the engines found the plane's right engine suffered more serious internal damage following bird strikes, as it was engulfed with big fires and black smoke. But the pilots switched off the plane's left engine, the report said, citing probes on the cockpit voice recorder, the flight data recorder and engine examinations.

Officials earlier said the black boxes of the Boeing jetliner stopped recording about four minutes before the accident, complicating investigations into the cause of the disaster. The cockpit voice recorder and the flight data recorder cited in the briefing report refers to data stored before the recording stopped.

The report didn't say why the pilots shut off the less-damaged engine and stopped short of saying whether it was an error by the pilots.

Pilots' union slams finding

Bereaved families and pilots at Jeju Air and other airlines lambasted the investigation findings, saying authorities must disclose the cockpit voice recorder and the flight data recorder.

"We, the 6,500 pilots at civilian airlines, can't contain our seething anger against the preposterous argument by the Aviation and Railway Accident Investigation Board that lost neutrality," the Korean Pilot Unions Alliance said in a statement Tuesday.

Unionized pilots at Jeju Air also issued a statement urging authorities to present scientific evidence to show the plane should have landed normally if it flew with the less-damaged engine.

The latest report focused only on engine issues and didn't mention other factors that could also be blamed for the crash.

Among them is the concrete structure the plane crashed into. It housed a set of antennas called localizers designed to guide aircraft safely during landings, and many analysts say it should have been made with more easily breakable materials. Some pilots say they suspect the government wouldn't want to prominently blame the localizers or bird strikes for mass deaths as the Muan airport is under direct management of the country's Transport Ministry.

The Aviation and Railway Accident Investigation Board and the Transport Ministry have offered no public response to the criticism.

Kwon Bo Hun, dean of the aeronautics college at the Far East University in South Korea, called the government's planned announcement "clumsy" because it didn't disclose evidence that supported its finding on the pilots. He said it only irritated "emotional parts of us that the investigation puts the whole blame on dead people."

A former Transport Ministry-turned-university professor reached by the AP said the engine investigation report must be "reliable" as it's based on an analysis of cockpit voice and flight data recorders that "don't lie." He spoke on condition of anonymity citing the delicate nature of the issue.