A top Republican senator is asking whether First Lady Michelle Obama's office played any role in last week's firing of former service program Inspector General Gerald Walpin. 

The concern, one of several surrounding Walpin's sudden dismissal, stems from the timing of a staff switch in the first lady's office. Just days before Walpin got the boot, the White House announced Michelle Obama's chief of staff would be appointed senior adviser to the agency Walpin was responsible for monitoring. Michelle Obama said at the time she and her outgoing staffer, Jackie Norris, would work closely going forward. 

With accusations now flying that the Walpin firing was politically motivated, the personnel change only adds to the list of questions Republicans have for the president. 

The White House insists the first lady had nothing to do with the firing. 

"She was not involved in any way in the decision to remove Mr. Walpin," White House spokesman Josh Earnest told FOXNews.com. 

Responding to criticism, President Obama on Tuesday offered a more detailed explanation for why he fired Walpin. In a letter sent through his special counsel, Norman Eisen, to leaders of the Senate committee that oversees AmeriCorps, Obama argued the agency's chief watchdog was so "confused" and "disoriented" at a board meeting that some questioned "his capacity to serve."

But Sen. Charles Grassley, R-Iowa, is looking for more documentation. 

He has sent a letter to Alan Solomont, board chairman for the Corporation for National and Community Service, asking that he provide all relevant correspondence from several offices and individuals. Included in that list was "contacts with officials in the Office of the First Lady." 

The letter did not elaborate on why the senator is seeking that information, but a Grassley aide confirmed that the Republican asked because of the recent staff switch in the first lady's office. 

Grassley's questions go far beyond that issue. He is among several Republican lawmakers who have sounded alarms about the firing and suggested President Obama canned Walpin in retaliation for investigating one of his supporters, Sacramento Mayor Kevin Johnson. Johnson was accused of misusing federal funds for an academy he founded. He eventually reached a settlement with the U.S. attorney's office.

Grassley, a frequent defender of government watchdogs, fired off thinly veiled accusations against the administration in his letter. 

"Inspectors general have a statutory duty to report to Congress. Intimidation or retaliation against those who freely communicate their concerns to members of the House and Senate cannot be tolerated," Grassley wrote. "This is especially true when such concerns are as legitimate and meritorious as Mr. Walpin's appear to be."

FNN