Steve Linick, the ousted Inspector General (IG) for the US State Department, was looking into whether Secretary of State Mike Pompeo had asked a staffer to run personal errands for him, media reports said.
In a report on Sunday, NBC News, citing two congressional officials, said that Linick, who was fired by President Donald Trump on May 15, was investigating whether Pompeo "made a staffer walk his dog, pick up his dry cleaning and make dinner reservations" for him and his wife, reports Xinhua news agency.
The officials said they are working to learn whether Linick may have had other ongoing investigations into Pompeo, according to NBC News.
CNN and The New York Times also reported the allegations on Sunday.
Trump said he's firing Linick because he hasn't had the fullest confidence in the official, who began his tenure as the State Department's watchdog in 2013.
A White House official reportedly said Pompeo recommended the move and Trump agreed.
The decision has triggered an inquiry from Democrats and scrutiny even from several Republicans.
Susan Collins, a Republican from Maine who co-authored a law asking the President to notify Congress 30 days prior to the removal of an inspector general along with the reasons for the move, tweeted on May 16 that the White House has not provided the kind of justification for the firing of Linick required by the law.
"I have long been a strong advocate for the Inspectors General," she said.
"They are vital partners in Congress's effort to identify inefficient or ineffective government programs and to root out fraud and other wrongdoing."
As Inspector General, Linick was responsible for, among other things, conducting administrative and criminal investigations of waste, fraud, mismanagement, and misconduct in the State Department.
He was appointed to the role in 2013 by then-President Barack Obama.
( With inputs from IANS )