Updated: 4:51 p.m.

With a “heavy heart,” embattled Illinois state Sen. Martin Sandoval announced his resignation late Wednesday amid a widening federal public corruption investigation.

The Chicago Democrat had his office and home raided by federal agents in late September as part of a multi-pronged federal probe into Commonwealth Edison’s lobbying activities, and potential wrongdoing in multiple southwest suburbs within his Senate district.

Sandoval, who has not been charged, told the Illinois State Board of Elections that his resignation from Illinois’ upper legislative chamber would take effect on Jan. 1.

“Respect for my constituents and the Senate has convinced me that this action is necessary in order to proceed without distraction to the important work that needs to be accomplished for working families throughout the state of Illinois in the future,” he wrote in his resignation letter.

Sandoval’s attorneys did not immediately respond to WBEZ’s requests for comment Wednesday afternoon.

Sandoval’s resignation marks a stunning fall from power for the one-time chairman of the Senate Transportation Committee, who helped Democratic Illinois Gov. JB Pritzker build statewide support for a $45 billion infrastructure program that lawmakers and the governor approved last spring.

During their Sept. 24 raids on Sandoval’s statehouse office and home, federal agents sought information about a litany of Springfield insiders and monied interests, including video gambling mogul Rick Heidner, road-paving magnate Michael Vondra and the state’s largest public utilities, Commonwealth Edison and its parent, Exelon Corp.

An FBI search warrant from the state Capitol raid sought information about four unidentified Exelon officials and any information related to utility rate increases.

Initially, Senate President John Cullerton’s office declined to make public the full contents of the FBI search warrant but relented after WBEZ threatened litigation under the state’s open-records law.

Sandoval, who took office in 2003, has remained out of the public limelight since agents carted out boxes of documents and electronic equipment from his office and home. He missed the entirety of the six-day fall legislative session that concluded in Springfield earlier this month.

State election authorities said Wednesday that a special filing period for potential successors to Sandoval will occur between Dec. 3 and 9, and the eventual winner of next November’s general election will serve out the remainder of Sandoval’s existing four-year term.

Between now and next November, an interim replacement will be named by Democratic committeemen in the 11th Senate District, which straddles the Stevenson Expressway and includes parts of Chicago, Cicero, Summit, Lyons and McCook.

Dave McKinney covers state government and politics for WBEZ. Follow him on Twitter @davemckinney. Investigative reporter Dan Mihalopoulos contributed.