Who killed Nancy?

In Oliver Twist, Nancy is a young girl who is a member of Fagin's gang. Fagin sells goods stolen by the young boys he recruits and trains. While she is a criminal, and Dickens alludes to her being a prostitute in his preface, she is depicted as both sympathetic and caring. She worries about Oliver and is fiercely protective of him. She gives information to Oliver's benefactor about Fagin's activities in order to prevent him from being kidnapped again, but Fagin's spies report on her betrayal. Though she never mentions Fagin's accomplice Bill Sikes by name, Fagin manipulates the information so that it seems as though she also turned on Sikes, who becomes so angry that he kills her.
It is Nancy's murder that ends up being the downfall of Fagin's gang. The investigation into her murder leads authorities to the gang's activities.