Tax returns are just the beginning: Trump biographer explains how subpoena could expose new financial crimes
Journalist David Cay Johnston is the founder of 'DC Report' (screengrab)

Prosecutors in Manhattan are now seeking eight years of tax returns and business records from President Donald Trump as part of a criminal investigation.

And as Pulitzer Prize-winning Trump autobiographer David Cay Johnston told CNN's Anderson Cooper on Monday, the investigation could ultimately reveal a lot more than just Trump's taxes.

"There have been a lot of questions throughout Donald's career about whether he was laundering money for people," said Johnston. "I and other people have taken apart some business transactions he did that make no sense from a normal business perspective, but would make a lot of sense if you were helping someone illicitly move money around."

"2012 — '11 and '12 — is when Donald was trying with a group of oligarchs to do a failed Trump Tower deal on the Caspian Sea," continued Johnston as an example. "And subsequently, the Kazakh government was looted of more than $10 billion, and the question has always been, was some of that money funneled in one way or another through the Trump organization? The tax returns may or may not tell you something about that."

"Tax returns are the beginning point for investigating where things are," added Johnston. "They're not the end point."

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