Who cast the killing curse on Fred Weasley?

Fred Weasley was not killed by the Killing Curse. He died during the Battle of Hogwarts, but the specific cause of his death is never explicitly stated.

The book, "Harry Potter and the Deathly Hallows", mentions that Fred was hit by "something" during the battle, and that George arrived just in time to see him die. This has led to much speculation among fans, but the exact cause of his death remains a mystery.