One month ago, Arthur A. Coia announced his retirement as president of the Laborers International Union of North America. This well-connected union boss, who has been an intimate of both President and Hillary Clinton, soon is expected to plead guilty to federal tax fraud charges. How and why the Department of Justice decided to limit its case to such a relatively narrow scope remains worthy of an investigation unto itself.

Mr. Coia, now 56, is something of a legend around Washington. While the FBI was alleging that he was "associated with and controlled by organized crime," and writers such as investigative reporter Eugene Methvin were calling him "a front man for labor racketeers who were looting pension and welfare funds," Coia was in full Bill-and-Hill hobnob:

- There was Hillary, five years ago this month, addressing Coia's union (one of this nation's largest for representing the building trades) in Miami Beach. She was fully aware of the federal probe; her speechwriter sent her a memo noting, "They're the mob."

- There was Bill, three months earlier, sending Art a warm note on becoming a grandfather the same day the Justice Department hand-delivered to the White House a draft of a civil racketeering lawsuit that it intended to file if Coia didn't quit.

- Prior to that, Coia had lent $100,000 to the president's inaugural committee. He'd also raised nearly $5 million for Democrat candidates and causes. Art became a regular dinner partner (state affairs included) and travel companion of the Clintons. He and the president traded pricey golf clubs.

A funny thing happened over the next few years. The Justice Department seemed to cave in ... to something . It gave the union a chance to clean up its own house, a most unusual move. Coia remained in power. Congress investigated the case in 1997, unearthing 120 notes between the Clinton White House and Coia. They show a clear cognizance of the alleged mob ties.

Coia claimed to have removed any influence of organized crime in the union's sweetheart deal with the government to cleanse itself. But by November 1998, The Washington Post reported, an independent union prosecutor alleged Coia had "knowingly permitted organized crime members" to influence Laborers International.

In another unusual move, an independent union hearing officer cleared Coia of any mob ties last March.

Coia said he retired to spend more time with his family and that he was tired of being hounded by prosecutors. The reality is, he cut a deal. Coia will plead guilty for failing to pay about $100,000 in taxes on the purchase of three Ferraris. And what a deal it is: Coia becomes president emeritus, a powerless position but one with the same salary as president, $250,000 a year.

Though serious enough, the tax fraud charges are a far cry from what federal investigators and prosecutors first targeted Coia.

But what really happened here? Did Arthur A. Coia get some kind of presidential dispensation? Why did the Justice Department effectively back off? Why didn't the Congress peel back this onion further?

The Coia case represents unfinished business. Coia's plea can't be allowed to represent the end of this matter. In fact, it should be considered a good place to start anew.