The Civil Aviation Authority of Vietnam said a person called the airline's office in Japan and said: "Flight VN5311 had better turn back to Narita or it will be shot down when flying over Tokyo Bay."
When the employee asked the man to repeat the message, he said: "I am preparing to shoot VN5311 when it flies above Tokyo Bay. It had better turn around."
The flight, which had taken off at Narita airport at 10:30 a.m., turned around at noon and landed in Fukuoka. It had 47 passengers and 15 crew members.
The threat was quickly reported to Japanese and Vietnamese authorities.
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, and it landed at 6:12 p.m.
Passengers were informed the aircraft had some "technical problems."
Tran Thi Ngoc of Hanoi's Me Linh District said at noon passengers were informed about a technical problem and turned around.
"At that time the plane had already [flown for quite a while], and when it turned around we felt a little worried."
At around 1 p.m. the plane landed in Fukuoka and the passengers had to wait for over an hour in two buses before returning to it.
"I saw some flight attendants hurrying about, but I did not think about a terror threat," Ngoc said.
Only when waiting for a Covid test at Noi Bai Airport in Hanoi did she come to know someone had threatened to shoot her flight down.
"We are all safe and feel lucky" and passengers comforted one another, she added.
Pham Thanh Hai of Thai Nguyen Province said he was not worried about the aircraft turning back due to technical problems since the pilots and flight attendants did not show any "special" reaction.
He too knew about the threat at Noi Bai Airport.
Minh Trung of Phu Tho Province was surprised to know about the threat and received messages of sympathy from relatives and friends after the incident went viral on social media.
VN5311 was the first regular flight between Vietnam and Japan after a hiatus of almost two years because of the Covid-19 pandemic.
Most of the passengers were Vietnamese workers in Japan who were returning home after a long gap.