The Vietnam Airlines flight from Tokyo to Hanoi on Jan. 5 had to turn back following a shooting threat by a person claiming to be American and speaking Japanese.
The person called the airline's office in Japan and said: "Flight VN5311 had better turn back to Narita or it would be shot down when flying over Tokyo Bay."
Japanese authorities have now identified a local man as the suspect, according to a source from Giao Thong newspaper, managed by the Ministry of Transport.
He had followed all information about the flight on the internet and made the phone call by himself.
Interrogation revealed the suspect had threatened to shoot down the Vietnamese plane without a specific motive or purpose.
The suspect appears to show signs of mental illness, the source said, adding Japanese authorities are investigating the case further.
By the time the threat was made, the aircraft, a Boeing 787, had already passed Tokyo Bay. There were 47 passengers and 15 crew members on board.
After discussions with Japanese authorities, the Civil Aviation Authority of Vietnam (CAAV) let the plane turn around and land at Fukuoka Airport for a security check.
The airport then arranged security and technical staff to review flight information and safety.
Japanese officials visited the airport and spoke to the flight crew and passengers to evaluate the situation. After finding no unusual signs and deeming it safe to proceed, the flight was allowed to leave Fukuoka for Hanoi, where it landed at 6:12 p.m.
Passengers were informed the aircraft had some "technical problems." All arrived safely in Hanoi and did not learn anything about the threat until landing.