On the afternoon of 6/16, based on the tower handoffs of the pings and drive time, Casey left the area of G&C's/Lee's ~4:10PM and arrived in the area north of Amscot ~4:33PM. Casey had placed her last call in the 2nd - albeit mini - "flurry" of calls that day to Cindy's cell phone about the time she passed by Amscot.
Then, there was a period of 28mins with no activity (Inactivity Period 1) on Casey's cell phone. We don't know what Casey was doing during this time frame, but, based on her next cell ping it appears she stayed somewhere around this area. Note that the following recognizable landmarks are located within the driving distance of the bookend pings that begin & end this 28min period.:
- Tony's apt.
- Gentiva
- Lexus
- Amscot
- CVS
- JB Park
Based on both of these bookend pings being from the same tower on the southern end of these landmarks, IMHO, it seems most likely Casey was nearer Amscot, one of the stores in this area, Gentiva or JB Park. Note, FWIW, that Casey's m.o. had been to drop Caylee @ Gentiva around Cindy's quiting time per several statements from Gentiva employees and George.
This first period of cell inactivity ended @ 4:53PM when Casey received a Facebook alert which pinged in the area described above.
After this Facebook alert was received there was a 64 minute period of inactivity
(Inactivity Period 2)that ended @ 5:57PM pinging at the location most likely to be near Tony's apartment with a system message that indicated either a voicemail or text message was waiting. Despite receipt this system message, Casey waited another 34 mins
(Inactivity Period 3)before initiating the 3rd "flurry" of calls that day to Cindy @ 6:31PM.
Up to this point, all of Casey's calls to Cindy had been attempts to get Cindy to answer the phone based on their duration being <10seconds. However, two calls to Jesse had durations of ~1minute...left vmail messages perhaps.
At 6:33PM Casey appeared to notice the system message indicating a voicemail or text was waiting and dialed her voicemailbox number.
IMHO, Casey had hoped this was a message from Cindy, etc, but, soon discovered it wasn't. Her call to voicemail lasted just under a minute and may have included checking Cindy's vmail receipt of messages Casey had left (an option from the vmail menu) and the text message that resulted from the Facebook alert...all of these can be done from the same call (I simulated myself this evening and took just over 30seconds to complete all three tasks). This series of actions provides one explanation for the system message(s) @ 6:34PM which arrived within seconds of the completion of the call Casey placed to her voicemailbox.
After this disappointment, there was another 32 minute period of cell inactivity
(Inactivity Period 4), until the 4th and final not-quite-a-"flurry" of calls began @ 7:06PM which sputtered and ended @ 7:21PM with calls to Amy. All of these activities pinged at a location most likely to be nearest to Tony's apartment.
Then...no cell activity...none... for the 33 mins
(Inactivity Period 5)...before Casey & Tony arrived @ Blockbuster IIRC ~7:54PM. Note that Blockbuster is less than 0.5 miles from Tony's apartment...so, some time must be allowed for Casey to have met up with Tony and for the pair to get to Blockbuster @ ~7:54PM.
IIRC, Cindy stated in her interview w/ the FBI that Casey called the evening of the 16th to explain that she & Caylee would not be home that evening. It appears to me that the 7:06PM call to G&C's home, which lasted just under 1.5 minutes, serves as the best candidate for this call. This would also be in agreement with the time needed for Casey to meet up with Tony and travel to Blockbuster.
Based on the above, it appears to me that, after attempting to repeat a drop of Caylee with Cindy @ Gentiva ~5PM 6/16, an upset Casey engaged in a change of plans in which she turned her phone off (or battery died) @ ~5PM just after the un-noticed receipt of the Facebook Alert @ 4:53PM, and turned her phone back on (or noticed & charger plugged in) @ 5:57PM which resulted in the system message being re-sent to her phone indicating she had a waiting message (only explanation for this pattern I can think of).
By 6:34PM Casey realized Cindy was not going to respond and Casey took the actions that ultimately cost Caylee her life. 
As noted before, medical death may not have occurred immediately, hence, the time of death can't be specified, however, the actions that resulted in Caylee's death appear to have been taken between 6:34PM and 7:06PM
