Their phones were turned off... Can they tell if they are turned off or just dead batteries? Why would they both be off at the same time.
Generally, a phone sends a signal to the tower when it is shutting down due to being turned off, telling the tower it can release the operational data it has stored for the phone. (Call data is retained for billing purposes.) If a phone is capable of sending the switch off signal, the next variable is if a tower is physically capable of receiving it. IIRC, a reporter said cell reception in the general area is spotty. An embankment certainly could block reception even in an area with strong coverage.
The
reference I've seen to their phones being shut off was from an uncle. People, including LE and the media, tend to default to saying a phone is shut off when it's unreachable. That's not necessarily technically true and may be obvious once investigative data is shared with the public, eg, pings from the phone simply ceased with no indication of why.
I'd assume no more at this point than their phones have not been responding.