WASHINGTON (CNN) - Law enforcement sources told CNN Thursday that the FBI has made a formal request to interview Bunnatine Greenhouse, the Army Corps of Engineers chief contracting officer who went public last weekend with allegations her agency unfairly awarded Kellogg, Brown and Root, a Halliburton subsidiary, no-bid contracts worth billions of dollars for work in Iraq.

The FBI interview has not yet taken place. Law enforcement sources also said that several documents have been gathered by the FBI related to the no-bid contracts.

However, a senior law enforcement official insists there is no federal investigation of the White House or Dick Cheney, who used to head up Halliburton (Research) before he became vice president.

The law enforcement sources said that the inquiry is in the very early stages and that "inferring anything at this juncture would be pointless." One senior law enforcement official said that following up such a public allegation of wrongdoing is nothing more than "due diligence."

Sources close to Greenhouse said she is willing to cooperate, but wants whistleblower protection to ensure no possible Pentagon retaliation to her going public.