Fred Weasley was killed by an explosion spell that many believes to be cast by Death Eater Augustus Rookwood
Highly improbable that it was Rookwood. Percy Weasley was with Fred when he died. The Blasting Curse was cast by Death Eaters from the grounds. Shortly thereafter, Percy spotted Rookwood chasing students down a corridor. Wanting revenge for Fred, Percy shouted Rookwood's name and chased after him.