An Appropriate Training Plan

The FBI was back at square one. Without Claire Conway's cooperation, they wouldn't be able to pin anything on the Numbers. 

Legally she didn't have to cooperate because they didn't even have enough proof against her to get a warrant. The word of one man—a previously convicted felon desperate for a reduced sentence—was not reliable enough for that. 

They tried other methods to see if there was any concrete evidence but always came up short. They tracked the email Derek had been sent to a public library computer. A guest login had been used so they couldn't trace a library card number either. 

That security footage didn't yield anything useful either. The person in question hid their face and hair from the cameras and wore baggy clothes so it was impossible to tell if they were male or female. 

Whoever had done this was smart. Either they had experience evading detection or had been told what to do by someone who did. Someone such as Six/John Sei.