CHANDLER, Ariz. — At first, the murder case against Orlando Nembhard seemed solid, as witness after witness came forward to say they saw someone that looked just like him brandish a pistol in February outside a crowded nightclub and fire on a young clubgoer, who later died.

But the case has turned shaky because someone who bears a striking resemblance to Mr. Nembhard — his identical twin brother, Brandon — was also at the scene in the early in the morning of Feb. 12.

Now the question facing the police and prosecutors is whether the right twin — Orlando, who goes by the nickname Slick, or Brandon, better known as Blizzy — was charged in the killing of Sir Xavier Brooks, 19, who had his hands up and was backing away from the gunman when he was shot in the abdomen.

With no forensic evidence pointing to the gunman’s identity, the police said, it comes down to the witnesses.