WATERVILLE — Sidney H. Geller said sometimes he lies awake at night and wonders how it happened.

Sarah Nale, who worked as a secretary for the prominent law offices of Geller and his son David, systematically stole more than $800,000 over the course of two decades from Geller’s real estate trust and from David Geller, according to court papers. Nale did so by diverting rent payments to herself and writing checks to herself for more than the amount recorded on the check stubs.

Now, federal prosecutors have reached an agreement with Nale to plead guilty and for her to pay back the money she stole. In the agreement to plead guilty, filed Monday in federal court, Nale has also agreed to waive her right to appeal. The agreement also proposes a prison sentence of not more than 21 months.

This deal will go before a federal judge for a final decision.

Court papers detail the actions prosecutors say Nale took. In addition to her work as a secretary, she also worked as a bookkeeper for the elder Geller, who was admitted to the Maine bar in 1963. Nale would deposit rents from tenants in the properties his real estate trust owns, which are mostly in Waterville on Silver Street, Main Street and College Avenue.

According to the documents, it’s not clear when alleged thefts began, but prosecutors say it was no later than March 2012 and it continued through September 2022.

In addition to diverting cash and checks, prosecutors say she collected rents and deposited them in a bank account for the law office and wrote checks to herself for money she was not entitled to. While it was her job to write out her own payroll checks, she is accused of writing them for more than she was owed, but recording only the amount she was owed on the check stub.

Attempts to reach David Bate, Nale’s attorney, were not immediately successful on Monday.

Geller, in an interview Monday with the Morning Sentinel, said he treated Nale like a daughter, and gave her bonuses and gifts as well as $1,000 to help pay for renovations to a home she and her family had bought.

He said there’s no way to tell when the alleged thefts started because financial records from before 2012 can no longer be found.

During this period, he said, he and his wife borrowed money to put their children through college, and in David Geller’s case, law school, so they could graduate without debt. David Geller has previously served on the Waterville Planning Board.

“What really irritated me is that at the end of the year, she would send how much the tenants paid, and I paid my taxes on those amounts,” Sidney Geller said. “I paid taxes on money I never received,”

In all, court papers say the amount stolen was $816,584. Of that, $687,780 was from the real estate trust, with $21,578 from Geller himself. The balance, $128,804, came from an account for David Geller.

“I asked the U.S. attorney if this is the biggest embezzlement he’s ever seen,” Geller said. “He said, no. There’s bigger ones.”