There are still a lot of unresolved factors, on whether Cheri Jo Bates ever entered the library or was abducted for the missing 3-4 hours and returned back to the scene. This however this should be regarded as the simplistic approach.
Cheri Jo Bates arrived at the library at 6.15 pm to pick up some reading material, parked her Volkswagen Bug on Terracina Drive and walked to the library annex, approximately 30 yards away. Robert Graysmith stated in his book "We figure Cheri Jo arrived at the RCC Library annex around six and went into the library. She had friends studying in the library, but none of them recall seeing her. We know she actually entered the library only because of three books on the Electoral College she checked out, which were found on the front seat of her car."
Assuming she entered the library, it had to be of short duration to avoid being noticed. Bearing in mind she arrived at the library at 6.15 pm, it is conceivable she had left as early as 6.20-6.25 pm. She attempted to start her vehicle, is offered assistance by an unknown male (the one who disabled it) and the irrevocable chain of events unfolded.
Cheri Jo Bates may simply have decided to walk back to the RCC Library to phone home, seek assistance or ask for a lift home once a suitable friend arrived. Had the 'good Samaritan' offering her help been a stranger, this would appear the sensible option, after all the library was only a mere 30 yards from her parked vehicle. If the good Samaritan was known to her, she may very well have accompanied him on the promise of a lift.
Depending on her interaction with the eventual murderer, this would shift the timeline about four hours before the widely accepted view, to around 6.30 pm - 6.45 pm. Sunset in Riverside was approximately 6.00 pm, so the light was ebbing away and her body lay there unnoticed for 12 hours, before Riverside City College caretaker Cleophus Martin discovered the stricken Miss Bates at 6.30 am on October 31st 1966.
But could Cheri Jo's body lie just off Terracina Drive for approximately 2 1/2 hours while people entered and exited the library that night without being spotted. It seems difficult to believe, but this has to be more likely than Cheri Jo Bates entering the cramped library at 6.15 pm and leaving at 9.00 pm without anybody noticing her. At least the library is illuminated.
Although not definitive, the autopsy results back up the earlier murder scenario, discussed here in Cheri-Jo-Bates-the-autopsy-findings. Stomach contents generally empty within 2-4 hours, but Cheri still had 100 ml in her stomach, a sizeable amount. Her last meal was at her Via San Jose residence around 4.30 pm, as testified by Joseph Bates and backed up by the constituent stomach contents. With the amount of food remaining in her stomach, her likely murder occurred no later than three hours after she had eaten, possibly as little as two hours, placing the time at 6.30 pm-7.30 pm.
The liver temperature of 26-28 degrees at autopsy (9.31 am) the following day, seems to back up the findings of the stomach contents, indicating a murder some 13-15 hours earlier. But considering the fact that Cheri Jo arrived at the library at 6.15 pm, the bottom end estimate of 6.30 pm seems the most likely.
The supposed killer would send a 'Confession' letter a month later, also backing up this earlier timeline. The author of the correspondence typed "Then I waited for her in the library and followed her out after about two minutes. The battery must have been about dead by then. I then offered to help. She was then very willing to talk to me. I told her that my car was down the street and that I would give her a lift home. When we were away from the library walking, I said it was about time she asked me, "about time for what ? ". I said it was about time for her to die."
If this person was the killer, and he followed her out of the library, or waited outside, then it had to be early, as again the only way for Cheri Jo Bates to have gone unnoticed by so many people, was to enter and leave the library relatively close to opening time, before the library filled up. The question is, how do you interpret the phrase "Then I waited for her in the library and followed her out after about two minutes." Did the killer wait for two minutes after Cheri Jo Bates left the library or did Cheri Jo Bates enter the library and leave two minutes later.
The fly in the ointment are the screams heard by eyewitnesses around 10.30 pm. But are these accurate and did they originate from Cheri Jo Bates. They are seemingly four hours too late and appear to conflict with all of the above. In addition, a female eyewitness apparently told police she was in the alleyway at 9.30 pm. where she saw a man smoking a cigarette. Where this account originated from, or whether it is has been fully authenticated is open to question.