The Army did not send a rescue team, believing all aboard were killed, so Ellison spent a year and a half in the Peruvian jungle with the native tribes before being discovered and returned to civilization.
When a sight zone-out occurs and Ellison is almost pasted by a garbage truck, he realizes that Sandburg is right that he needs someone to watch his back and bring him out of zone-outs.
Five years before the series opens, Ellison, then an Army Special Forces officer, was the sole survivor of a helicopter crash in the jungles of Peru.
Theorizing that Ellison's extended stay in the jungle activated his latent abilities, Sandburg wants to study Ellison for his research.
Ellison just wants the heightened senses to go away, but Sandburg convinces him that the thing to do is to get them under control.