A phone being shut off or the battery dying (and it powering down), will prevent a phone from pinging. What usually happens (from following lots of murder cases on here), is that the phone will ping one final time, giving authorities that last known location.
Newer model iPhones do have a feature where your phone can be located when powered down, which requires someone with an Apple device to travel near your missing device (you do have to opt into this in your settings).
ETA: We don't know the model of device (as far as I know), nor do we know how long it would survive in water. Newer model phones can typically be submerged for much longer than devices in the past, which would be damaged within seconds.
Her friend said that they went back to Kiely's car to charge their phones at one point, so there's a decent chance she didn't have much battery to begin with.
With Apple's Find My app, you can hunt down your lost or stolen devices.
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