This sounds like it occurred at the chemical feed station for the Cooling Tower on the roof, since it was on the 45th floor.
In NY, most common biocides are Sodium Hypochlorite and if fed in a common line with other cooling water chemicals, a reaction occurs, especially in low flow situations.
A similar reaction happened in Boston a couple of years ago, resulting in the Cooling Tower burning down. Sodium Hypochlorite was the cause.