During a Police Line-Up, the Arresting Police Officer Places the Suspect

During a police line-up, the arresting police officer places the suspect under custody in a room with
five other individuals. The arresting police officer then enters a room where a witness waits and opens
a one-way window to reveal the lined-up individuals. The arresting officer then asks the witness to
identify which of the individuals he or she recognizes as the perpetrator. This is an example of a blind
situation, because the witness doesn't know which individual is the actual suspect. How could this
situation be made double blind?