Complex sentence: Kerik was in his office when the first attack occurred and arrived at the base of Tower I about three minutes before United Airlines Flight 175 hit Tower II, showering him and his staff with debris from the burning tower and plane.
Split into simpler sentences: Kerik was in his office when the first attack occurred. He arrived at the base of Tower I about three minutes before United Airlines Flight 175 hit Tower II, showering him and his staff with debris from the burning tower and plane.