Air officials are trying to find out how a man apparently smuggled explosives onto a plane by hiding them in the heel of his shoe.
He was overpowered by passengers and crew when he lit a match while the plane was over the Atlantic Ocean.
It was an American Airlines flight from Paris, France to Miami in the US.
The plane made an emergency landing in Boston escorted by F-15 jets.
An airport spokesman said when they X-rayed the man's shoe, holes had been drilled and there were explosives hidden inside.
He said it could have been a "very serious" incident.
The man was reportedly travelling on a fake British passport.
French authorities are trying to work out how he slipped through the security which has been tightened up since the 11 September attacks.
There have been fears that members of Osama Bin Laden's al-Qaeda terrorist network will try to carry out more acts of terrorism and US air chiefs have warned people to be on their guard.