It has been often mentioned before that the Zodiac Killer may have been British based upon certain words he used such a ‘shall’, etc, but it is entirely plausible that although he spoke in an American accent, he could of had British parents, thereby he heard British language usage. This may explain the lack of contact during three successive years when he was at his peak ie between December 20th 1968 to July 4th 1969, December 20th 1969 to April 20th 1970 and October 27th 1970 to March 13th 1971. He never made contact during these consecutive years in January or February, with notable gaps of 7 months, 4 months and nearly 5 months. Was he visiting relatives in England or Britain, as opposed to interstate, particularly over Christmas and the new year.
After this he may have left the US and returned briefly in 1974, just after Christmas and again in 1978 and 1990. Then he failed to return for good.