Complex sentence: Using the preserved fingerprints from the wallet, John identifies the Nightingale as Jack Shepard (Shawn Doyle), a former detective who, in the original timeline, died from a medical error the same night Frank died.
Split into simpler sentences: Using the preserved fingerprints from the wallet, John identifies the Nightingale as Jack Shepard (Shawn Doyle), a former detective. In the original timeline, Shepard died from a medical error the same night Frank died.