I wanted to take a closer look at this topic. We know Libby had a phone and presumably other devices on which she used social media and other programs (MP said in either Down the Hill or Scene of the Crime that the night the girls went missing, he returned to their home and gathered up iPads, iPods, laptops, and any device he could think of with internet capability and delivered them to LE to be searched for signs of where the girls might have gone). In Scene of the Crime, AW said that BP was a step ahead of her and already had passwords for Libby's devices. Supposedly Abby had a tablet but not a cell phone. So what would LE have been able to legally obtain pertaining to data, including social media conversations, regarding these devices?
I found this recent article that talks about what LE can legally do:
The police want your phone data. Here’s what they can get — and what they can’t.
The short answer is, it depends. But chances are with the right legal paperwork (warrants etc), the FBI can get it.
Can law enforcement get third party data from your phone? Yes, and it doesn't even need physical possession of your phone if you're using some type of cloud service. If you are sending or receiving DMs on Twitter or Instagram for example, and that info is stored on the provider's servers, with a subpoena, warrant or court order, they can likely get access. The quote from an expert in the article was: "Anything that a provider has that they can decode, LE is getting it."
Note: if LE wanted to see, for example, text messages that the girls were exchanging with a friend, and that friend agrees to show them to LE, a warrant isn't even needed. If the friend refuses, a warrant can be pursued to get access.
What if the phone is password protected? Supposedly BP had Libby's passwords but let's assume she didn't. Courts have ruled that the 5th Amendment protects individuals from being compelled to reveal their passwords to devices or encrypted programs. However, if you are dead, your 5th Amendment rights die with you. Once Libby was discovered to have been murdered, LE could legally use passcode cracking programs if needed. If LE has the necessary search warrants, these can be put into motion.
What about the issue mentioned in this thread with regard to the San Bernardino terrorist's phone, where the FBI had to sue Apple to try to force them to unlock it? As this article describes, the FBI actually had ways to get into the encrypted phone all along, but wanted to establish a legal precedent to force phone manufacturers to make it easy for them to access devices in criminal cases:
Revelations on the FBI’s Unlocking of the San Bernardino iPhone: Maybe the Future Isn't Going Dark After All
As the article states, the FBI was, in fact, able to use one of its contractors to break into the phone before the court case with Apple was settled. So they were able to get the access they needed regardless, though they probably weren't thrilled that they had to advertise that they had the capability to do this.
The FBI Operational Technology Division has two units that work on issues pertaining to data like this: the Cryptologic and Electronic Analysis Unit, which works mostly on criminal cases, and the Remote Operations Unit, which uses network exploitation techniques and works largely on classified (government) cases. They collaborated on the San Bernardino criminal case.
We heard in the early days of this investigation that it was briefed all the way up to the office of then-director of the FBI James Comey. So IMO if there were FBI resources that could have helped interpret the girls' social media and other encrypted digital activity, I think that all of these resources would have been used.
IMO LE in this case knows exactly what the girls were doing online and with whom AND they know whether this was a factor in the crime.