In the absence of a confirmed explanation for Robert Nichols’s behavior, speculation filled the void. Investigators themselves examined several hypotheses, and internet researchers generated others. It is worth reviewing what was considered and what was established.
**The Zodiac Killer theory** circulated for years before and after the 2018 identification. The argument rested on several overlapping observations: Nichols had lived in northern California during the period when the Zodiac was active (roughly 1968–1969, though the Zodiac’s crimes extend into the early 1970s); he had a technical background consistent with some descriptions of the Zodiac; he had electronics and drafting skills that some analysts associated with the nature of the Zodiac’s encrypted communications; and the thirteen-character string “Robert Nichols” has been noted to match the thirteen-character format of one of the Zodiac’s letters. A composite sketch associated with the Zodiac was said by some to resemble Nichols.
None of these observations constitutes evidence linking Nichols to the Zodiac crimes. They are pattern-matching on characteristics shared by a large number of people. Elliott’s 2018 statement that the question was deferred to California authorities was accurate and appropriately cautious. The Zodiac case has its own DNA evidence from a stamp that the killer reportedly licked; if that DNA is ever conclusively extracted and compared, it could either implicate or eliminate specific candidates. No such comparison establishing or excluding Nichols has been publicly announced.
**The D.B. Cooper theory** also circulated online. D.B. Cooper is the pseudonym used by a person who hijacked a Northwest Orient aircraft in November 1971, demanded and received a ransom, parachuted from the plane over the Pacific Northwest, and was never found. The connection to Nichols was never developed beyond the observation that Nichols was a man who had disappeared and that D.B. Cooper was also a man who had disappeared. Investigators did not give this theory significant public attention.