Lana and her clone.
Unbeknownst to anyone else, Lana found out that Lex had created a clone (model 503) of her in a 33.1 facility at the Reeves Dam as well as, in order to escape from both Luthors, decided to fake her death and worked out a complicated plan.
Lana had gotten into the mail truck that came in between Lionel and her car.
She provoked the discussion with Lex so this might give her the perfect alibi and made Lionel believe that she really was leaving Smallville to avoid suspicion and to have a potential witness for everything that would happen. She put the Model 503 in her car and then blew it up in front of Lionel, making it appear that Lex had murdered her.