Security was tight at the Sochi antidoping laboratory during the day. But it lapsed at night, when tainted urine samples were swapped. Leon Neal/Agence France-Presse – Getty Images

Grigory Rodchenkov, the antidoping laboratory director, said that each night a sports official sent him a list of athletes whose samples needed to be swapped.

Athletes also sent photos of their doping control forms to help identify which urine sample were theirs.

Upon receiving a signal, usually after midnight, Dr. Rodchenkov went to Room 124. The room was officially a storage space, but he and his team had converted it into a laboratory.

Room 124 was next to the official sample collection room where the bottles of urine were kept.

Room 124 Room 125 Sochi Olympic antidoping laboratory Secured area First floor Entrance Entrance Room 124 Storage space where Dr. Rodchenkov and his team worked. Hole Room 125 Official urine sample collection room. Urine sample bottles were passed through a hole between the two rooms. Room 124 Room 125 Secured area First floor Sochi Olympic antidoping laboratory Entrance Room 124 Storage space where Dr. Rodchenkov and his team worked. Hole Room 125 Official urine sample collection room. Urine sample bottles were passed through a hole between the two rooms. Room 124 Room 125 First floor Secured area Sochi Olympic antidoping laboratory Entrance Urine sample bottles were passed through a hole between the two rooms. Room 124 Storage space where Dr. Rodchenkov and his team worked. Hole Room 125 Official urine sample collection room. Room 124 Room 125 Sochi Olympic antidoping laboratory Secured area First floor Entrance Entrance Room 124 Storage space where Dr. Rodchenkov and his team worked. Hole Room 125 Official urine sample collection room. Urine sample bottles were passed through a hole between the two rooms. Diagram by Jeremy White

A colleague in the collection room passed the urine samples through a hole in the wall near the floor. The openings were covered with white plastic caps. The opening on the collection room side was also concealed by a small faux-wood cabinet during the day.

View of the hole from the “storage space” where Dr. Rodchenkov and his colleagues worked. View of the hole from the official urine sample collection room.Photographs by Grigory Rodchenkov, via Bryan Fogel, Icarus Documentary Film

The urine sample bottles, manufactured by Berlinger, a Swiss company, were designed so that they could not be opened without breaking the cap once the bottle had been sealed. When it is time to test the urine sample, the cap is removed by breaking it into two parts with tools or machines sold by Berlinger.

Unique 7-digit code Security cap locks by pushing down while screwing it on. Spring pushes the metal ring against the glass teeth. Metal ring with teeth prevents bottle from being opened. Glass teeth lock against the metal teeth. Unique 7-digit code Unique 7-digit code Security cap locks by pushing down while screwing it on. Spring pushes the metal ring against the glass teeth. Metal ring with teeth prevents bottle from being opened. Glass teeth lock against the metal teeth. Unique 7-digit code Diagram by Yuliya Parshina-Kottas

In Room 124, Dr. Rodchenkov received the sealed bottles through the hole and handed them to a man who he believed was a Russian intelligence officer. The man took the bottles to a building nearby. Within a few hours, the bottles were returned with the caps loose and unbroken.

Each cap is imprinted with a unique number to match the bottle, so no other cap can be substituted. The bottle is five inches tall and two inches wide. Tony Cenicola/The New York Times

Dr. Rodchenkov’s team emptied and cleaned the bottles with filter paper and filled them with untainted urine collected from the athletes months before the Olympics.

They would then add table salt or water to balance out any inconsistencies in the recorded specifications of the two samples. Depending on what an athlete had consumed, two urine samples taken at different times could vary.