A massive ammonia leak in the Seaport killed a man Wednesday night. According to the Boston Fire Department, the leak was reported just before 6 p.m. at Stavis Seafoods Warehouse at 7 Channel St. It was more than three hours before crews were able to stop it. Fire officials said an employee was trapped in a stairwell near the second floor, overcome by ammonia fumes. They attempted a rescue attempt, but were unsuccessful because of the fumes. Everyone else who was inside the building is safe and accounted for.