Complex sentence: Felt believed Gray's methods were an unnecessary distraction and showed a lack of leadership, and was sure he was not the only member of the FBI's leadership who disapproved of Gray's methods, particularly among those who had served under Hoover.
Split into simpler sentences: Felt believed Gray's methods were an unnecessary distraction and showed a lack of leadership. He was sure he was not the only member of the FBI's leadership, who disapproved of Gray's methods, particularly among those who had served under Hoover.