EDITOR’S NOTE: Stream CNN Films’ Top of the World, an intimate look at Windows on the World, the legendary restaurant atop the World Trade Center that became one of America’s most celebrated restaurants. The film tells the story of its remarkable history, and the lives lost there on September 11, 2001. Available now on CNN All Access.

On the morning of September 11, 2001, Sekou Siby should have been 107 floors above Manhattan, supervising the breakfast cold- food station at the iconic Windows on the World restaurant.

Instead, he was home in bed beside his pregnant wife. A few days earlier, Siby had traded shifts with Moises Rivas, an Ecuadorian musician in his 20s who played in a band on weekends. Rivas needed Sunday off, so Siby covered for him. On Tuesday, Rivas returned the favor and took Siby’s place in the kitchen.

On that fateful morning, Siby was awakened by a frantic phone call from a coworker’s wife and turned on his TV to see the North Tower engulfed in flames. He later learned that Rivas was one of the more than 2,700 people killed at the World Trade Center in the deadliest terrorist attack in American history.

For Siby, who is originally from Ivory Coast, the scale of the loss was impossible to comprehend.