The crash of a fighter jet in the U.S. city of San Diego, which resulted in the deaths of a Korean immigrant family, was found to have been caused by both mechanical and human errors.

On December ninth, a fighter jet crashed into a residential area and killed Yoon Dong-yoon’s wife, their 15-month-old and two-month-old daughters and his mother-in-law, a Korean citizen who was visiting the family to assist her daughter with the raising of her baby.

According to an investigation by the U.S. Marine Corps, the fighter jet's ground crew did not properly follow emergency protocol while giving instructions to the pilot who was experiencing engine problems.

An oil leak was also found to have been neglected for months.

U.S. authorities and Yoon are expected to start negotiations regarding compensation for the accident.