South Korea's Air Force announced Wednesday it has found no evidence of any mechanical failure related to the deadly crash of an F-15K fighter jet earlier this month.

It suggests the possibility of human error or an unknown "environmental factor" behind the accident that killed two pilots.

The aircraft crashed into a 839-meter-high mountain in Chilgok, North Gyeongsang Province, on April 5.

The Air Force has been looking into the exact cause. Based on the early analysis of the flight data recorder and an on-site probe so far, it said it has been confirmed that there was no defect in the jet itself.

"Accordingly, (we) are investigating various possibilities, including environmental and human factors," it said in a statement.

It added it plans to resume the operation of dozens of F-15K fighters, grounded shortly after the crash, Thursday. (Yonhap)