Complex sentence: Even if he did land safely, agents contended, survival in the mountainous terrain would have been all but impossible without an accomplice at a predetermined landing point, which would have required a precisely timed jump--necessitating, in turn, cooperation from the flight crew.
Split into simpler sentences: Even if he did land safely, agents contended that survival in the mountainous terrain would have been all but impossible without an accomplice at a predetermined landing point. This would have required a precisely timed jump--necessitating, in turn, cooperation from the flight crew.