It wasn't completely empty.
This is a critical element of the FBI's version of the event.
A completely empty gas tank will not ignite.
A very low tank will, given the right ignition.
A full tank will not.
Also, the fuel used is very hard to light.
If you don't believe me, ask experienced commercial 747 pilots.
They will tell you that they don't need all the fuel, mainly on the tailwind flights. When flying against, a different story.
If you have ever flown a plane, gas needed for trip there is different than what is needed for trip back.