The mayor of a northern Illinois city has resigned after being named one of nine people charged in a police investigation into soliciting a prostitute.

Rick A. Olson resigned as Sandwich mayor on Monday, three days after being charged with solicitation of a sexual act.

Sandwich City Clerk Denise Ii says Olson's letter was read late Monday at a City Council meeting. Because the city does not have a vice mayor, the council will appoint a mayor pro-tem to run the meeting.

"I sincerely apologize for the shame I have brought and I pray the city will continue to prosper," Olson's letter read in part.

The 66-year-old Olson had been mayor of Sandwich since 2013. He served as the city's police for the prior 16 years.

Authorities say the investigation that led to Olson's arrest began in December 2016, with the arrest of three women for prostitution. The investigation led to a second phase that targeted men seeking services.