Follow the emails. Papadopoulos admitted a Russian contact told him in April 2016 they had thousands of Clinton emails that contained dirt. Although Trump and his team now claim that Papadopoulos had no influence on the campaign, after Papadopoulos's April meeting with the Russian email contact, on July 27, 2016, Trump stated, “Russia, if you’re listening, I hope you’re able to find the 30,000 [Hillary Clinton] emails that are missing,” he said. “I think you will probably be rewarded mightily by our press. Let's see if that happens. That will be next.” (Curiously, he then didn’t hold another press conference for the rest of the campaign.) 

No one outside of the campaign knew about Papadopoulos' meeting with Russians regarding emails prior to Trump's July statement. At the time, no one outside the campaign understood why Trump  was soliciting Russian help in making them public. Now we do.