Tristan Snell is a former assistant attorney general for New York state, where he helped lead the investigation and prosecution of Trump University. The opinions expressed in this commentary are his own. View more opinion articles on CNN.

It is difficult to overestimate the importance of Allen Weisselberg, the Trump Organization's chief financial officer, being subpoenaed to testify before the grand jury empaneled in the Michael Cohen investigation in the Southern District of New York, as first reported by the Wall Street Journal.

An attorney for the Trump Organization, Alan Futerfas, told The Washington Post , "Mr. Weisselberg is a bookkeeper who simply carries out directions from others."

But Allen Weisselberg is not just the Trump Organization's bookkeeper. He is arguably the single most indispensable person in the Trump Organization, other than Trump himself.

In the Trump University case , which I helped lead, the investigation revealed that amid the sprawling tentacles of the Trump Organization, everything leads back to two people: Allen Weisselberg and Donald Trump. Trump repeatedly denied wrongdoing in the case and claimed that he could have won at trial, but agreed to settle the case

Weisselberg was the master of all things financial. He knew where every dollar in the organization came from, and he knew -- and controlled -- where every dollar went. He was Trump's right-hand man.

Here are just some of the many facts that came to light with regard to Trump University:

Only five authorized signatories for the Trump University bank account existed: Donald Trump, Ivanka Trump, Donald Trump Jr., Eric Trump, and Allen Weisselberg. This is also the same select inner circle of the Trump Foundation now being investigated by the New York attorney general.

According to the New York state attorney general office's filings in the Trump University case , Weisselberg personally reviewed all of the finances of Trump University, a subsidiary of the Trump Organization, every month. Trump University's controller, Steven Matejek, testified that he had to produce the financials every quarter for Weisselberg and meet in person to review them.

For more than a year, Matejek did not even have access to view Trump University's checking account, and even when such access was granted, he was still not allowed to issue any payments or transfer any money -- those powers were reserved for Weisselberg. Weisselberg also had managerial authority over Michael Sexton, Trump University's president.

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Weisselberg signed virtually all of the checks or Trump University himself. The only exceptions came when Donald Trump wrote checks to himself, allegedly ripping off of thousands of people.

So the real question now is: what does Weisselberg know? What could he reveal to investigators?

Weisselberg likely has information regarding the Trump Organization's tax returns, and according to other reports , Donald Trump's personal tax returns as well. So if Donald Trump's biggest secret is that he actually is far more or less wealthy than he claims, Weisselberg likely has information on that, too.

The news about Weisselberg means that the investigations into Donald Trump are nowhere near winding down. Prosecutors usually only pursue higher-level figures in an organization when they believe they have the goods and are looking to corroborate existing evidence, fill in the narrative, or continue working up to the head of the organization.

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What is clear is that when it comes to these investigations, the addition of Weisselberg to the mix likely means things are still only getting started.