The theory of meeting up with someone else in the spot she was near:
1. So far we hear she was walking there on the last camera, on her own
2. The LE would have video of either all people walking on the same or nearby cameras soon after, they could identify and question them all and take their DNA..etc
3. If a car stopped there for a prearranged meeting, they could also see on alternate cameras on either side of the area, based on speed and time to enter/leave from one camera to the next, if a car took longer and would had either slowed down in this period or possibly stopped. This is very ascertainable.
4. Subpoena all cell phone activity in the area. Heard they were all ready doing this. I think they do not need to know who called, I think all cell phones give off their signal to the proximate towers just by picking up a signal. This info should be stored. So anyone with a cell phone that is on in the area, bam, got ya.
Both 2/3/4 are also ascertainable in finding a random and would in fact provide many suspects in the area if not all of them, so I find it very hard to believe they will not catch this perp, I think either method even if a large net because of how dispersed the cameras and or towers, that they could very well find the perp this way.
Also, the theory of who did she contact, to rule out too many of the above possible suspects, based on either:
a. calls/texts she sent, which they could subpoen all records from wtv telco she used
b. emails/ims, they could subpoen from wtv provider based on the accounts they KNOW OF
c. for all accounts they are unaware of, well yes without having the actual "device" the app was used on, it is hard to determine what other accounts she had.
They could do the following to determine other unknown accounts:
- check all access to other emails she may have used on another device back home that Steve could provide to them, the sessions will be there even if deleted, there are ways to still trace them.
- If she did not use any other devices back home, they can still contact the main provider she used back home i.e verizon if she had an account with them on her ipad or Samsung, and ascertain all apps she downloaded or were sessions were seen being used from the device over their network, and then contact each co and subpoen records that will show all accounts that accessed their services on her account or even just with the same device ID.
- If she only used wifi on one of her devices, or downloaded an app specifically while overseas, they could still spend some more research bandwith and see with wtv provider her device used by her device ID on their servers what her activities were and with what apps, they contact these app providers to ascertain what other accounts she may have had. Then all angles would have been taken into account.
There are ways of determining all her accounts even without the devices, they are just more lengthy, but I assume the FBI or even LE has tech forensics to do this work.