A heroic Marine has told how he managed to stop a manic passenger from killing everyone onboard a flight from Japan to Houston, Texas.

Jody Armentrout, a sergeant major who has served in the Marines for over 20 years, was sitting in the emergency exit row on All Nippon Airways Flight 114 from Tokyo to Houston on Saturday when he said he noticed a man acting strange.

Trained to remain vigilant at all times, Armentrout said he watched as the man take his backpack into one of the bathrooms, then came out and went to another bathroom on the flight - and then a third.

'He came out of that one and began pacing up and down the aisle, so that just threw my radar on,' the 50-year-old Marine told NBC News.

Soon, he said, he saw the man eyeing the emergency exit door next to him.