the truck
I've read that the writers originally planned this to be two films. The first would've revealed Marie as the killer in only the last few minutes. The writers were convinced by Luc Besson to change it to the way it is. The second would've been the same events, but the true version instead of Marie's story.
I see that as good and bad. On one hand, the movie obviously wasn't received well enough to justify a sequel, and who would really want to sit through it? However, had it gone that way, maybe the sequel would've actually at least offered a satisfactory explanation for the twist.