Last Updated May 30, 2015 7:39 AM EDT
Law enforcement sources tell CBS News the case against former House Speaker Dennis Hastert, who's charged with violating banking laws to pay hush money and then lying about it to the FBI, involves sexual misconduct with a male student.
It dates back more than 30 years to when Hastert was a high school teacher and wrestling coach in Yorkville, Illinois, just west of Chicago.
CBS News' Julianna Goldman reports former colleagues on both sides of the aisle say they were caught off guard by the allegations.
In a statement Friday night, House Speaker John Boehner said, "The Denny I served with worked hard on behalf of his constituents and the country. I'm shocked and saddened to learn of these reports."
Law enforcement sources said the misconduct mentioned in court documents refers to sexual misconduct dating back more than 30 years.
From 1965 to 1981, Hastert was a popular teacher and wrestling coach in Yorkville.
He worked there before joining the House of Representatives and going on to become the longest-serving Republican speaker of the House.
In a statement Friday, the Yorkville school district said it was "first made aware of any concerns regarding Mr. Hastert when the federal indictment was released" on Thursday.
Prosecutors allege the former House speaker had agreed to pay $3.5 million to someone they refer to only as "Individual A" "to compensate and conceal his prior misconduct against" that person.
From June 2010 through April 2012, Hastert made 15 $50,000 bank withdrawals. That caught the attention of the banks, and after he was questioned by bank representatives, he began withdrawing cash in increments of less than $10,000, allegedly to avoid further scrutiny.
In all, he withdrew $1.7 million, handing the money over to "Individual A" at pre-arranged meeting places and times.
In 2014, the FBI questioned Hastert about his withdrawals, and he allegedly lied, telling agents he was storing cash "because he did not feel safe with the banking system."
The news has stunned former colleagues, like Sen. Mark Kirk of Illinois.
"I was shocked, completely shocked," Kirk said.
It was Hastert's clean image that led Republicans to tap him as speaker in 1999 after ethics scandals, but he resigned after Republicans lost control of the House in the 2006 elections and in the wake of another major scandal, when then-Rep. Mark Foley resigned after it was revealed that he sent sexually explicit communications with House pages.
"We have had so many of our political figures, in both political parties, state and federal level, who have been accused and prosecuted for their deeds in office; it's embarrassing," Democratic Sen. Richard Durbin said.
Hastert could appear in court for his arraignment as early as next week. The charges against him are not related to the allegations of sexual misconduct. Hastert has not responded to numerous requests for comment.