TULSA, Oklahoma - Once named one of the top attorneys in Tulsa, Robert Nichols has submitted his resignation to the Oklahoma Bar Association.

The State Supreme Court suspended his license to practice when the bar began investigating accusations of him stealing clients' money - hundreds of thousands of dollars’ worth – and it may have been happening for more than a decade.

The Oklahoma Supreme Court will have to approve Nichols' resignation, and that could take a month.

The bar association is proactively leading the investigation, and local attorneys said that is rare.

Three computers, a few files and some Remington sculptures are all that's left of the Law Office of Robert Nichols. His assets are frozen while the Oklahoma Bar Association investigates him for running what the general counsel calls a Ponzi scheme.

Nichols is accused of settling a case, keeping the client's settlement check and cashing it for himself. Then, when another case is settled, paying back the previous clients and keeping the remaining money.

Former client Ralph Mackey said it happened to him when Nichols faked his signature on his almost $200,000 settlement check and cashed it.

"Obviously didn't look like my signature at all. My heart just sunk. My stomach turned," Mackey said.

He has since filed a lawsuit against Nichols.

We reached out to Nichols and his attorney, but have not heard back.

An affidavit details five other cases in which the bar association is investigating Nichols for keeping clients’ money.

In those five cases, Nichols is accused of collecting more than $235,000 in client funds, and "keeping [kept] all the money for himself."

That's not all. The bar association said dozens of lawyers have been hired to handle Nichols' 40 pending cases - and at least ten of those cases involve misappropriated funds.

The bar association and a Tulsa law firm, Frederic Dorwart, are helping alleged victims get their money back.

General Counsel would not say whether the bar is also investigating Nichols' partner, Brian Curthoys.