U.S. ATTORNEY Ronald C. Machen Jr. this week outlined to a local audience some of the difficulties his office has faced in investigating the 2010 campaign of D.C. Mayor Vincent C. Gray (D). People lying. Documents shredded. Potential witnesses sent out of the country. He then rather pointedly noted the refusal of D.C. Attorney General Irvin B. Nathan to turn over documents that are “highly relevant to our inquiry.”

We hope Mr. Nathan responds by dropping his claim of attorney-client privilege and cooperating more fully with federal authorities. The client that needs to be served is the public. Its interest lies in getting to the bottom of wrongdoing in the 2010 election. If no crimes have been committed other than those exposed already, the public needs to know that.

Federal investigators are seeking e-mails and other documents relating to the 2011 decision by the Gray administration to pay a $7.5 million settlement to a health-care firm owned by Jeffrey E. Thompson. Mr. Thompson is a central figure in the federal probe; he has not been charged but is alleged to have financed a $653,000 secret campaign that helped elect the mayor. Mr. Gray, who has acknowledged that the probe may affect his decision on whether to seek reelection next year, has denied any wrongdoing.

City officials have said repeatedly that the settlement with D.C. Chartered Health Plan was aboveboard, in the city’s financial interest, agreed to by career lawyers and approved by federal officials. Mr. Nathan said the city has given prosecutors nearly 20,000 documents; officials, including Health Care Finance Director Wayne Turnage, have freely talked to investigators. Mr. Nathan said he won’t sacrifice the principle of attorney-client privilege for what he has essentially characterized as a fishing expedition.

But, as Mr. Machen bluntly said Wednesday, “It’s not like we’ve been looking at this for three years and there’s no there there. I mean, there’s there there.” Four people associated with the 2010 mayoral campaign, including longtime friends of Mr. Gray, have pleaded guilty to felonies.

Investigators’ interest seems focused on why there was a change in thinking among District officials about paying Chartered’s claims from that of the previous administration. A procurement officer issued an opinion Aug. 11, 2010, denying the claims; Chartered’s appeal, filed Sept. 24, 2010, was pending when Mr. Gray took office. Among those whom investigators want to interview is Ganayswaran Nathan, a deputy to Mr. Turnage who was deeply involved in the settlement. Mr. Nathan, no relation to the attorney general, was caught up in the controversy that surrounded hiring decisions in the early months of Mr. Gray’s administration.

Mr. Gray “wants Nathan placed in Health Care Finance,” former Gray confidante Lorraine A. Green wrote in a Feb. 9, 2011, e-mail to the mayor’s chief of staff, according to The Post’s Mike DeBonis and Nikita Stewart. Two days later, interim human resources director Judy Banks wrote to Mr. Turnage: “THIS is the top priority appointment the Mayor wants us to make in Health Care Finance. . . . Huge political capital riding on this one.” The e-mails became public during the D.C. Council’s scrutiny of hiring, prompted by Sulaimon Brown’s claims that he got his job, also in Health Care Finance, as a reward for his political activities.

A Gray spokesman said the mayor had long known Ganayswaran Nathan and recommended him based on his Medicaid experience, including work for the city’s chief financial officer; Mr. Turnage said he hired Mr. Nathan for his skills. Mr. Nathan told us he had supported Mr. Gray’s election but said he had “no idea” what the reference to political capital was about. He said he is perfectly willing to talk to investigators: “I have done nothing wrong. ”

The settlement raised eyebrows, with some council members wondering why the city was rewarding a firm that had been accused of bilking it. Continued interest by federal investigators indicates that questions remain. We urge the city to accede to the U.S. attorney’s request for relevant documents.