The police chief said the heist remains the unsolved mystery o | 📣 Quiz Masters 🇬🇧

The police chief said the heist remains the unsolved mystery of the decade.

Answer: The unnecessary word here is < ...unsolved... >.
TIP: Unsolved means "without explanation or answer", and a mystery is "a strange and unexplained event".

The police chief said the heist remains the unsolved mystery of the decade.