An official responsible for monitoring how federal funds for volunteerism are spent told FOX News he was fired by President Obama for doing his job, and suggested it was payback for investigating the alleged misuse of grant money by the Sacramento mayor, an Obama backer.

Gerald Walpin, who until last week was the chief internal watchdog for AmericCorps and other service programs, suggested "political pressure" was behind his ouster. He said he worried the action will have a "chilling effect" on other inspectors general. 

Obama gave little explanation for the decision, writing in a letter sent Thursday that he no longer had the "fullest confidence" in Walpin. 

The president's decision followed Walpin's investigation into the alleged misuse of federal grants by Sacramento Mayor Kevin Johnson, a former NBA star and an Obama supporter who heads a nonprofit education group. 

"I am the victim of being fired because I was doing my job and doing it properly," Walpin told FOX News Tuesday. He said he could not let concern for "political pressure" interfere with his staff's pursuit of the investigation -- noting that it was his staff, not him personally, that pursued the inquiry. 

He also defended the findings, saying AmeriCorps requested the investigation in the first place. 

Walpin said the probe revealed that Johnson "misused" AmeriCorps volunteers for "personal purposes," by having them help in political campaigns and even wash his car. 

"They never disputed it whatsoever," Walpin said. "And indeed the agency itself found that our statements were correct and our findings were correct." 

Walpin, though, drew criticism elsewhere. After Walpin referred the matter to prosecutors in late 2008, the local U.S. attorney's office questioned Walpin's findings, saying they seemed overstated and did not accurately reflect all the information gathered. 

Johnson's St. HOPE Academy released a statement last week after the firing, saying Walpin's allegations were "meritless" and not motivated by an "honest assessment" of the program. 

The U.S. attorney's office nevertheless reached a settlement, under which Johnson and the academy reportedly were ordered to repay about half of nearly $847,000 in federal grants they received through AmeriCorps. 

Walpin, though, said the agreement was signed "behind our back" and Johnson never was required to repay any money personally. 

With his firing, Walpin said he's worried about the "chilling effect" it will have on his staff as well as other inspectors general who are supposed to be free to investigate independently concerns surrounding the agencies to which they're assigned. 

He also raised concerns that Obama sidestepped a law he co-sponsored that requires the president to give 30 days notice, and a reason, for removing an IG. 

Rep. Darrell Issa, R-Calif., ranking Republican on the House Oversight and Government Reform Committee, wrote White House Counsel Gregory Craig a letter on Monday requesting additional documents, explanations and e-mails surrounding the firing. 

In the letter, he said Obama may have "violated" the law by giving an explanation that was "insufficiently vague" and immediately terminating Walpin. 

Though the Obama letter technically included a 30-day window, Issa said his committee's investigation revealed that Walpin was given scant notice. He wrote that Walpin was given one hour by White House staff to either resign or be fired. Walpin would not resign.

Issa noted growing speculation that the firing was "politically motivated" and retaliatory. 

Sen. Charles Grassley, R-Iowa, who has been speaking up for the integrity and independence of federal inspectors general, also wrote a letter to Obama last week saying he was "deeply troubled" by the appearance that Walpin was given an "ultimatum" without sufficient notice. 

"Inspectors general were designed to have a dual role reporting to both the president and Congress so that they would be free from undue political pressure," he wrote. "This independence is the hallmark of all inspectors general and is essential so they may operate independently, without political pressure or interference from agencies attempting to keep their failings from public scrutiny," he wrote. 

Grassley agreed with Walpin, saying "it appears he has been doing his job." 

Craig wrote Grassley saying Obama's decision was "carefully considered." 

The Associated Press contributed to this report.