In a telephone interview on Wednesday, Mr. Christie said he chose Mr. Ashcroft and the others for the monitoring assignments because they had impeccable legal credentials and he knew and trusted them.

“It’s really important that the working relationship between this office and the monitors is very, very close,” he said. “I can’t tell you how much work we do with these monitors.” He said he had selected Mr. Ashcroft to work with Zimmer, the largest of five companies in the criminal investigation, because “I knew he was somebody who understands these issues and would be taken seriously by the company as an authority figure.”

Mr. Christie has disputed accusations raised by Democratic lawmakers in New Jersey that it was a conflict of interest for him to direct large, no-bid contracts to former colleagues and friends, but he has referred those questions to the Justice Department in Washington.

Department officials said they had no formal comment but noted that the monitoring agreements were not given only to Republicans and that Mr. Christie’s recommendations of outside monitors in other large corporate investigations had been praised.

Although he was a prosecutor in the Bush administration, Mr. Kelley has registered as a Democrat in the past. Mr. Kelley, who has done legal work for The New York Times, did not respond to e-mail messages on Wednesday. Mr. Samson and Ms. Yang did not return phone calls.

The dollar value of the contracts obtained by Mr. Kelley, Ms. Yang and Mr. Samson is unclear, since the medical-supply companies they are monitoring have not revealed those details, suggesting that they are smaller than Mr. Ashcroft’s.

Under the settlements with the Justice Department, the companies negotiate the fees with the monitors themselves, a situation legal scholars say has the potential for abuse because companies might be overly generous to encourage leniency.