U.S. ATTORNEY Ronald C. Machen Jr. has come under increasing criticism for the pace of his investigation into the 2010 campaign of D.C. Mayor Vincent C. Gray (D). We, too, would like to see a resolution of these troubling questions. But federal prosecutors have faced considerable obstacles in getting needed information.

The latest example of a speed bump is the refusal by D.C. Attorney General Irvin B. Nathan to give prosecutors e-mails and other documents related to the payment of millions of dollars to a city contractor who is at the center of the probe. Federal prosecutors, The Post’s Ann E. Marimow and Mike DeBonis reported, are examining a $7.5 million settlement with D.C. businessman Jeffrey Thompson in the first year of Mr. Gray’s administration. The payment represented an about-face from the reasoning advanced by the previous administration. Mr. Thompson, who has not been charged with a crime, is alleged to have financed an illegal “shadow campaign” that helped to elect Mr. Gray. The mayor also has not been charged with a crime.

Mr. Nathan cited attorney-client privilege in rejecting Mr. Machen’s request for the information. An unusually pointed exchange ensued, with the attorney general calling Mr. Machen’s line of questioning “preposterous.” Mr. Nathan said there was nothing improper about the settlement, which, he noted, had been approved by federal Medicaid officials.

Mr. Nathan offered a compromise late Thursday, by which documents would be turned over to the chief judge of the U.S. District Court for the District of Columbia for private inspection and a determination of whether they are indeed privileged. We appreciate Mr. Nathan’s concern for attorney-client privilege and his desire not to set a bad precedent for future administrations. But that concern seems to us to be outweighed by the public’s interest in having prosecutors get to the bottom of the story.

Even as Mr. Nathan expresses impatience with federal prosecutors, his proposal for a judge’s review would introduce further delay into the federal investigation. We urge him to cooperate with the U.S. attorney.