A two-term Oklahoma state senator pleaded guilty on Thursday to multiple charges wire fraud and filing a false tax return in connection with a scheme to embezzle more than $1.8 million from a nonprofit agency.

State Sen. Rick Brinkley, a Republican, pleaded guilty to the six federal charges, including five counts of wire fraud and one count of subscribing to a false tax return. Brinkley resigned from his state Senate seat Thursday morning.

Prosecutors say Brinkley stole $1.8 million from the Better Business Bureau of Tulsa, where he had worked for 15 years.

Defense attorney Mack Martin said the two-term senator recently spent two months at an inpatient treatment facility for a gambling addiction. Martin told The Associated Press that Brinkley decided to plead guilty to federal wire fraud charges Thursday because it was in the best interest of his family and friends.

Republican Senate President Pro Tem Brian Bingman issued a statement Thursday shortly after Brinkley submitted a letter of resignation, effective immediately.

Bingman says that with Brinkley's resignation and a special election having been called, the Senate can put the matter behind it "and look forward to conducting the business of the state."

The Associated Press contributed to this report.