WE HAD thought we’d seen the ultimate in gall when D.C. Attorney General Irvin B. Nathan refused to give federal investigators documents needed in their investigation of government corruption. But then Mayor Vincent C. Gray (D) told Washington City Paper that prosecutors should “put on the table” evidence of anything untoward about the District’s $7.5 million settlement payment to a company owned by the alleged financier of the “shadow campaign” that helped elect Mr. Gray.

It was rich for the mayor to demand that federal investigators produce evidence when it was his attorney general who had balked at requests from U.S. Attorney Ronald C. Machen Jr. to do precisely that — and when Mr. Gray has declined to meet with prosecutors. It is untenable for Mr. Nathan not to realize that his office could best serve D.C. residents by fully cooperating with federal investigators.

An exchange of letters between Mr. Nathan and Mr. Machen, released this week, suggests the two sides may be approaching a compromise over what information the city will share with prosecutors concerning the controversial 2011 settlement with D.C. Chartered Health Plan over reimbursement rates for dental care.

Jeffrey E. Thompson, who headed the company, is under federal investigation involving allegations that he financed an illegal shadow campaign on Mr. Gray’s behalf in 2010. He has not been charged.

Mr. Nathan has vigorously defended the settlement as equitable and aboveboard. But as Mr. Machen wrote in a sharply worded letter, Mr. Nathan has acknowledged he doesn’t have a full picture of the events. “The investigation,” Mr. Machen wrote Nov. 25, “has identified connections and communications between D.C. Chartered’s agents and District officials that you likely do not know about.”

Mr. Nathan became involved after administration officials decided to settle, reversing a decision made months earlier by the previous administration. He first learned of the matter when it was brought up during a mayor-council breakfast — “a break down in the process,” according to a Nov. 20 e-mail to us from Health Care Finance Agency Head Wayne Turnage. Mr. Nathan insisted the matter be reviewed and, in the end, he signed off on settling, albeit with a payment reduced from $10.2 million to $7.5 million.

Questions remain about this payment. Mr. Nathan should respect the limits of his knowledge and give investigators the access they need. He must also understand, as Mr. Machen noted, that he cannot be the final arbiter of what documents are necessary to the investigation “because of your relationship with and obligation to the District officials whose actions are under review.” Mr. Nathan says he is protecting the lawyer-client privilege. In this case, he and the mayor have a greater responsibility not to interfere with a legitimate criminal investigation.