By Joe Barrett

CHICAGO -- Exelon Corp.'s Commonwealth Edison unit agreed to pay a $200 million fine for a long-running bribery scheme in which the utility gave jobs and contracts to associates of Illinois House Speaker Michael Madigan, the state's most powerful politician for decades, the U.S. attorney's office for the Northern District of Illinois said Friday.

Under a deferred prosecution agreement, the Illinois utility must continue to cooperate with the investigation until it is complete, the U.S. attorney said. It wasn't immediately clear whether Mr. Madigan, who has held the post of speaker for all but two years since 1983 and is also head of the state Democratic Party, was a target of the investigation.

A spokesman for Mr. Madigan didn't immediately return calls for comment. A spokesman for the U.S. attorney declined to comment on whether Mr. Madigan was a target of the probe.

Illinois Gov. J.B. Pritzker, a fellow Democrat, said Mr. Madigan must resign if the allegations are true.

"The speaker has a lot that he needs to answer for, to authorities, to investigators, and most importantly, to the people of Illinois," Mr. Pritzker said during a stop in suburban Chicago, according to the Associated Press.

Exelon said it acted swiftly to conduct its own investigation when it learned of the allegations and determined that a small number of senior ComEd employees and outside contractors were responsible.

"Since then, we have taken robust action to aggressively identify and address deficiencies, including enhancing our compliance governance and our lobbying policies," the company said.

The U.S. attorney said that ComEd awarded the jobs and contracts, some of which involved little or no actual work, to associates of Mr. Madigan when the company had substantial matters before the legislature, including legislation affecting the rate-making process. Mr. Madigan, the U.S. attorney said, controls what legislation is voted on in the state house and has substantial sway over lawmakers.

Court documents refer to Mr. Madigan as Public Official A and later identify Public Official A as the Speaker of the Illinois House.

ComEd also said that it had named a member of its board at the request of Mr. Madigan, retained a law firm at his request and accepted a certain number of students from his Chicago ward into its internship program, the U.S. Attorney said.

As head of the state Democratic Party, Mr. Madigan wields a campaign chest that essentially bankrolls the campaigns of many state legislators, said Dick Simpson, a political-science professor at the University of Illinois, Chicago, and a former Chicago alderman.

The indictment is part of a probe of members of the Illinois legislature, including efforts by some suburban lawmakers to lobby local officials to install red-light cameras on behalf of companies that make the cameras, said Mr. Simpson, who is also co-author of the book "Corrupt Illinois: Patronage, Cronyism and Criminality."

The investigation has revealed an "intertwined shakedown and bribery operation," Mr. Simpson said, noting that based on overall convictions of public officials since the 1970s, Chicago is the most corrupt metropolitan region in the country, and that Illinois is the third-most corrupt state, based on per capita convictions of public officials.

Four previous Illinois governors, including most recently Democrat Rod Blagojevich and Republican George Ryan, have been convicted of crimes during or after their time in office.

Write to Joe Barrett at joseph.barrett@wsj.com