WASHINGTON — The Pentagon's top investigator is resigning amid accusations that he stonewalled inquiries into senior Bush administration officials suspected of wrongdoing.

Inspector General Joseph Schmitz told staff members last week that he intends to resign as of Friday to take a job with the parent company of Blackwater USA, a defense contractor.

The resignation comes after Sen. Charles Grassley, R-Iowa, sent Schmitz several letters this summer informing him that he was the target of a congressional inquiry into whether he had blocked two criminal investigations last year.

Grassley also accused Schmitz of fabricating an official Pentagon news release, planning an expensive junket to Germany and hiding information from Congress. Schmitz is the senior Pentagon official charged with investigating waste, fraud and abuse.

"I am writing to inform you that I intend to conduct an oversight investigation into allegations that you either quashed or redirected two ongoing criminal investigations last year," Grassley, chairman of the Finance Committee, wrote in a July 7 letter.

Grassley's office said Friday that the inquiry was continuing.

"Many questions need to be answered," said Beth Levine, a Grassley spokeswoman.

The inspector general's office denied any connection between Schmitz's resignation and the inquiries, saying Schmitz previously announced his intention to leave after Bush's first term of office ended.

A spokesman declined to comment on the allegations contained in Grassley's letters.

The first of the criminal investigations in which Schmitz allegedly intervened involved John Shaw, the former deputy undersecretary of defense for international technology security.

Shaw, the subject of a series of articles in the Los Angeles Times last year, tried to manipulate a lucrative contract in Iraq in 2004 to favor a telecommunications company whose board included a close friend, according to whistle-blowers who worked for the U.S.-led Coalition Provisional Authority in Iraq.

At the time, Shaw had signed an unusual agreement with Schmitz that provided him with some investigatory powers. Shaw told U.S. officials in Iraq that he was using his agreement with Schmitz to conduct a series of investigations during a trip to Iraq in December 2003. The results of those investigations were later used in his effort to push for contracts of companies tied to his friends and their business clients, according to the whistle-blowers.

Shaw, who was forced out of office last year, has denied wrongdoing.

Schmitz referred the accusations to the FBI over the protests of senior criminal investigators in his office, who had found "specific and credible evidence" of wrongdoing by Shaw, according to Grassley's letter. The FBI has not placed a high priority on the investigation, which has stalled, according to one person with knowledge of the case.

Schmitz then helped craft a news release in which his office denied ever investigating Shaw, according to Grassley's letter.

The second investigation in which Schmitz allegedly interfered involved Mary Walker, general counsel for the Air Force.

Grassley said in the July 7 letter that the accusation appears to involve Walker "lying under oath," possibly during investigations of either the Air Force Academy or Boeing.

The Air Force Academy has been rocked by controversies the past few years, including allegations of rape of female cadets. An Air Force procurement officer was sentenced to nine months in jail after giving Boeing preferential treatment in the negotiation of a $23 billion deal to lease refueling planes, and later taking a job with the company.

Grassley wrote that senior criminal investigators had "specific and credible evidence" regarding Walker, but the case was "allegedly shut down for unexplained reasons and possibly referred to the FBI." Grassley's letter said Schmitz was a "personal acquaintance" of Walker.

The Air Force said Friday that Walker had been cleared of wrongdoing by Schmitz's office. Walker could not be reached for comment.

Grassley also expressed concern that Schmitz had withheld information from Congress on the Boeing investigation. Schmitz came under criticism earlier this year when he redacted the names of top White House officials in delivering his report on the Boeing deal. He first submitted his report to the White House for review.

Finally, Grassley reprimanded Schmitz earlier this year for planning to take a ceremonial trip to Potsdam, Germany, that would have cost taxpayers $16,000. Schmitz later canceled the trip.

Schmitz will go to work for Prince Group, the parent company of Blackwater USA, as the chief operating officer and general counsel. Schmitz formally recused himself in June from any cases involving Blackwater, a private security company with millions of dollars in contracts in Iraq.