Complex sentence: Upon reaching IX, Starr and Bigman are warned away by the head of the project, Commander Donahue, who argues the men on the project are angry at investigation and may threaten Starr's safety.
Split into simpler sentences: Upon reaching IX, Starr and Bigman are warned away by the head of the project, Commander Donahue. He insists the men on the project are angry about being constantly investigated, and he can't guarantee Starr's safety.