Mercer parent company has this on wiki:

At the time of the 2001 September 11 attacks in the United States, the corporation held offices on eight floors, 93 to 100, of the North Tower of the World Trade Center.[21] When American Airlines Flight 11 crashed into the building, its offices spanned the entire impact zone, floors 93 to 99.[22] Everyone present in the company's offices on the day of the attack died as all stairwells and elevators passing through the impact zone were destroyed or blocked by the crash; the firm lost 295 employees and 63 contractors.[1]