From the article:
An earlier letter, addressed to four different newspapers, contained hidden identifiers — including his military units — that pointed to Rackstraw, now 74 and living in the San Diego area. Rackstraw, who could not be reached for comment, was previously investigated and cleared by authorities of being Cooper, but he remains the most likely suspect in the elusive case.
See posts 15, 25, 26, and 28.