Michael Isikoff, co-author of "Russian Roulette," says the Helsinki meeting between Trump and Putin will be "unprecedented." Isikoff talks with Brian Stelter about the mysteries surrounding Trump's ties to Russia. They also discuss the media's coverage of the Robert Mueller probe. Isikoff says there are questions that need to be asked, but journalists should be careful not to "over-speculate."To learn more about how CNN protects listener privacy, visit cnn.com/privacy