Complex sentence: The latter, mystified at this strange behavior, does an investigation of his own and soon finds out what has happened, but by this time Simpson has gotten involved in a running shootout with the heavily armed Stark (Xander Berkeley).
Split into simpler sentences: The latter, mystified at this strange behavior, does an investigation of his own, and soon finds out what has happened. By the time he has found out, however, Simpson has gotten involved in a running shootout with the heavily armed Stark (Xander Berkeley).