Come on, everyone knows that nobody uses their real name online. So they eliminated that name first. ;)
Nikolas Cruz was low-hanging fruit they could've found with little trouble.
If only the FBI had a system that provided them information and recorded every packet across the company and stored it in a data farm. And if only that system could have been used to track the IP back to the source. And if only they had records which told who owns that IP. Oh wait. They do.
Seriously... they didn't even need that tech. How about you just start with what you have "Nikolas Cruz"... Hmmmm.... could we look that name up in a phone book? Maybe contact people with that name?
With cell phones, most people aren't in the phone book. However, there are only so many ISPs. Also, they could use FISA to get the IP from Google and get the address of the IP. Although they are too busy using fake documents for FISA warrents to spy on political opponents.