In a letter to the San Francisco Chronicle postmarked October 13th 1969, the Zodiac Killer threatened to "wipe out a school bus some morning. Just shoot out the front tire + then pick off the kiddies as they come bouncing out". A telephone threat to bomb a school bus and kill children from somebody claiming to be the Zodiac Killer was targeted at Santa Rosa on October 15th 1969, prior to the details about "wiping out a school bus and picking off the kiddies" had been released by the newspapers. There were reports of a possible gunshot being heard by a Santa Rosa bus driver on October 17th 1969. These threats on schoolchildren brought a massive response from school officials and law enforcement, who laid out extensive measures to combat the perceived threat by the Zodiac Killer. Extra security to the buses also came in the form of police vehicles shadowing the schoolchildren to and from their respective schools.
On October 21st 1969, the San Francisco Chronicle released an article entitled Fear Rides the Yellow Bus, stating "A climate of fear hangs over the Napa Valley. The fear is over "Zodiac" and his terrifying boast to "wipe out a school bus". No one takes the threat lightly, Zodiac has already struck here. The task of trying to insure the safety of 10,000 youngsters who ride the Napa Valley Unified School District is immense". It was two days later that Daniel Williams started receiving menacing phone calls from a caller claiming to be the infamous Zodiac Killer. The caller stated that "he intended to kill several persons" and on one occasion said he "had gone to a Martinez school in search of victims but left when he found police there". Bearing in mind the heightened security around school buses and therefore school premises, this was entirely plausible. This could easily have been interpreted as an empty threat from another hoaxer, but it's a fair assumption that the phone caller and arsenic poisoner were one and the same. There were probably many idle hoaxers latching onto the Zodiac case during this period of uncertainty, but I doubt many were prepared to back up their threats with murder, as was the case in the attempted poisoning of Daniel Williams. If this person was prepared to kill a school teacher, then it's not beyond the realms of possibility he was prepared to kill schoolchildren in Martinez (where Daniel Williams lived) or the city of Richmond, where Daniel Williams taught at Salesian High School.
The Zodiac Killer, by poisoning the soft drink of Daniel Williams with arsenic, having previously stated he "had gone to a Martinez school in search of victims but left when he found police there", must have concluded that his visit to a Martinez school, in accompaniment to Daniel Williams being a Richmond school teacher, was more than enough to occupy law enforcement on several fronts. He may have thought that if the police didn't take the threat of murder seriously, then they would likely take nothing seriously.
With the failed poisoning on November 2nd 1969, the Zodiac Killer wrote in the Bus Bomb letter "up to the end of Oct I have killed 7 people", possibly indicating he hadn't yet finished with Daniel Williams (or possibly wanted us to believe that). When the phone caller to Daniel Williams mentioned his visit to a Martinez school, he concluded that police would never catch him because he was "too smart for them". In the Bus Bomb letter on November 9th 1969, the Zodiac Killer stated "The police shall never catch me, because I have been too clever for them". The Zodiac Killer was certainly a clever wordsmith, but was he responsible for the attempted murder of Daniel Williams?