CSI: Crime Scene Investigation (2000)

S4E23 Bloodlines

On her way home from work, a casino staff member is brutally assaulted, but she manages to survive and points out her attacker. However, a DNA test unexpectedly clears him of wrongdoing, prompting Grissom to grapple with the reliability of evidence. Meanwhile, Grissom finds himself helping a colleague who finds trouble on the wrong side of the law.