Complex sentence: During this period, he told friends that he was receiving death threats by phone from either CIA, or Defense Intelligence Agency (DIA) operatives--at the time, he related that he was not sure which of the two agencies was behind the threats.
Split into simpler sentences: During this period, he told friends that he was receiving death threats by phone from either CIA, or Defense Intelligence Agency (DIA) operatives. At the time, he related that he was not sure which of the two agencies was behind the threats.