Complex sentence: At around 8:20 a.m. on September 16, Alexis arrived at the Naval Sea Systems Command headquarters in a rental car and used a valid security pass to enter the Navy Yard.
Split into simpler sentences: At around 8:20 a.m. on September 16, Alexis arrived at the Naval Sea Systems Command headquarters in a rental car. He used a valid security pass to enter the Navy Yard.