Complex sentence: After successfully evacuating the majority of Morgan Stanley's 2,687 employees, he went back inside the building, where he was last seen on the 10th floor, heading upward, shortly before the tower collapsed.
Split into simpler sentences: After successfully evacuating the majority of Morgan Stanley's 2,687 employees, he went back inside the building. He was last seen on the 10th floor, heading upward, shortly before the tower collapsed.