The FBI told the ABC affiliate in Detroit that Umar Farouk Abdulmutallab wore the explosive underwear for three weeks, in order to get used to it and make sure he could get it through security.
Agents say the act of wearing the underwear for so long caused the bomb not to detonate. Instead, it caught fire and severely burned him.
Abdulmutallab pleaded guilty to trying to blow up a Northwest Airlines plane from Amsterdam over Detroit on Christmas Day in 2009. He is in prison in Colorado.