ST. LOUIS — St. Louis County's top elected official resigned Monday after being indicted on corruption charges accusing him of trading political favors for campaign donations.

A federal grand jury indicted County Executive Steve Stenger on charges of bribery, mail fraud and the theft of honest services, the U.S. attorney in St. Louis announced in a news release. He could face up to 20 years in prison on each count, if convicted.

Stenger's attorney, Scott Rosenblum, confirmed the resignation but he declined to comment further. Stenger pleaded not guilty during a brief court hearing and was released without bail. The St. Louis County Council on Monday night chose Councilman Sam Page, a Democrat, to replace Stenger.

"I have determined after much thought that it is in the best interest of our County and my family that I resign as St. Louis County Executive effective immediately," Stenger wrote in a brief resignation letter.

Stenger, a Democrat elected to his second term in November, is accused of ensuring that donor John Rallo and his companies obtained contracts with the county and received other favors. Stenger also is accused of ensuring that an unnamed company obtained a state lobbying contract from the St. Louis Economic Development Partnership, and taking actions to conceal the illegal conduct.

The U.S. attorney's office said Stenger sought to "secretly use his official position to enrich himself through soliciting and accepting campaign contributions from individuals and their companies in exchange for favorable official action, and for individuals and their companies to enrich themselves and their companies by secretly obtaining favorable action for themselves and for their companies, through corrupt means."

Rallo was not named in the indictment. Federal prosecutors say Stenger took actions to ensure county contracts for two Rallo-owned companies — Cardinal Insurance and Cardinal Creative Consulting. He also allegedly took action to ensure that Rallo's Wellston Holdings LLC obtained options to buy two properties in the town of Wellston that were held by the county's Land Clearance for Redevelopment Authority. The voicemail on a phone for Rallo was full and not accepting messages on Monday.

Asked if Rallo might face charges, Assistant U.S. Attorney Hal Goldsmith said the investigation is ongoing.

St. Louis County is Missouri's largest county, with a million residents.

Despite reports of the pending investigation against him, Stenger narrowly defeated Mark Mantovani in the August Democratic primary and easily defeated Republican Paul Berry III in the general election.

He has been an avid supporter of the "Better Together" proposal that would merge St. Louis city and the county if voters approve a November 2020 ballot measure. The initial plan even called for Stenger to become the metro city's first mayor. That proposal was revised in March amid reports of ongoing investigations of Stenger.

Investigations in recent months by the Post-Dispatch have raised concerns about county contracts going to Stenger's political donors. The St. Louis County Council began an ethics investigation that included asking state and federal investigators to look into the county's lease of office space at a renovated shopping mall owned by developers who gave $365,000 to Stenger's campaign.

Stenger has repeatedly denied wrongdoing.

In January, the St. Louis Economic Development Partnership's board of directors forced out Sheila Sweeney, who was appointed by Stenger to run the partnership. Her removal followed Post-Dispatch investigations that, among other things, raised questions about its procurement practices and the awarding of contracts to Stenger's campaign donors.

Two key Stenger aides, Chief of Staff Bill Miller and senior policy adviser Lance LeComb, left their jobs this month.

Stenger is the second executive of a large Missouri county to be indicted in recent years. A federal judge last year sentenced former Jackson County Executive Mike Sanders tomore than two years in prison for misusing campaign funds for trips, fine wine and political activities. Much of Kansas City is in Jackson County. Sanders resigned from his county executive job at the end of 2015.