Not sending a written paper trail should be covered in Criminal Surivial Strategies 101. Whether it's telegraph, snail mail, email or WhatsApp, this is just sloppy criminal work. For example Steven Cohen avoided an insider trading conviction by making sure nothing bad ever had his name on it. If you needed to discuss something secret, you talked to him in person.
Even though Manafort was associated with foreign government figures it took being associated with the U.S President to be investigated. It didn't matter before... having a paper trail was ok, no one cared.
Background: https://www.google.com/amp/s/amp.ft.com/content/efda2ca2-ec6...