Vermont Attorney General T.J. Donovan has settled a fraud investigation over the billing practices of Jonathan Gruber, one of the architects of Obamacare who was advising the state over its a now-abandoned single-payer plan.

Under his $450,000 contract with the state, Gruber was supposed to submit monthly invoices describing the work he did and how much he was billing for his work. The attorney general's office said Gruber filed at least two invoices that were false that pertained to work performed by a research assistant who was working for him.

Gruber, a Massachusetts Institute of Technology economist, has denied committing fraud. To resolve the case, he has agreed to forgo additional payments the state owed him, a total of $90,000 in outstanding payments. The case was settled under the state's Civil False Claims Act, which the attorney general's office determined he had violated.

The attorney general's office began its investigation into Gruber's invoicing practices after it received a referral from State Auditor Doug Hoffer.

Donovan's office said the supporting documents Gruber provided didn't reflect the actual hours that the research assistant worked or the hours he spent on the project. According to state documents, the attorney general's office determined that in order to meet the hours that were billed then, the assistant must have worked 12 hours a day during the first period billed and more than 16 hours a day for the second period billed. The documents go on to state that the assistant was salaried but should have specified the actual number of hours worked.

An inquiry to Gruber was not immediately returned.

Former Gov. Peter Shumlin, a Democrat, looked to create a single-payer system in Vermont, called Green Mountain Care, but abandoned the plan after determining it would be too costly.

Gruber came under fire in 2014 for comments he made about how the "stupidity of the American voter" helped Congress pass Obamacare.

"Lack of transparency is a huge political advantage," he said at a healthcare conference. "And basically, call it the stupidity of the American voter or whatever, but basically that was really, really critical for the thing to pass."

Former President Barack Obama, when questioned about these comments at the time, initially called Gruber "some adviser who never worked on our staff." Gruber, however, was paid $400,000 by the government to help Department of Health and Human Services estimate healthcare costs.

He also was instrumental in writing the healthcare plan in Massachussetts when Mitt Romney, a Republican, was governor. The plan, sometimes dubbed "Romneycare," served as a model for Obamacare.