Seems that they were communicating via LinkedIn.
It may be that LE couldn't get the appropriate warrant for LinkedIn
messages until they had probable cause (from the spy pen)?
BBM
Except in limited emergency situations, law enforcement agents seeking information about member accounts must be made through formal U.S. legal procedures, such as subpoenas, court orders, and search warrants.
Please note that certain types of member data, including messages, invitations and connections, have a high bar for disclosure and can only be disclosed pursuant to a valid search warrant from an entity with proper jurisdiction.
LinkedIn Law Enforcement Data Request Guidelines | LinkedIn Help