THE INDICTMENT of Zacarias Moussaoui for conspiracy in connection with the Sept. 11 attacks is an encouraging sign that the enormous criminal investigation that followed the attacks is making progress. Mr. Moussaoui was not charged with document fraud or overstaying his visa but with being an al Qaeda member actively involved in the conspiracy to, as the indictment put it, "kill and maim persons within the United States," a plot that led to "the deaths of thousands of persons on September 11, 2001."

The indictment describes a circumstantial case against Mr. Moussaoui, who was picked up on immigration charges before Sept. 11. But the facts it alleges, if proven, are far from innocuous. The government contends that Mr. Moussaoui was trained at an al Qaeda terrorist camp in Afghanistan and that after coming to this country, his behavior closely tracked that of the Sept. 11 hijackers. Like them, he did flight training. He bought some of the same flight training videos as did hijacker Mohammed Atta. Like Mr. Atta, he was interested in crop-dusting technology. Like the other conspirators, he bought knives and joined a gym.

And he allegedly received money from the same man in Germany who wired money to another of the hijackers.

The fact that the indictment was filed in U.S. District Court -- not before a military commission -- also is encouraging. Mr. Moussaoui appeared to be among the more likely domestic candidates for military justice under the White House order allowing for such tribunals. He is not a citizen, and some of the evidence implicating him came from an overseas intelligence service. That the government considers it possible to release enough information about his case to gain a conviction without seriously harming national security is a welcome vote of confidence in the civilian justice system.

Mr. Moussaoui is innocent until proven guilty. That's a hard principle at times such as these, and getting him a fair trial won't be easy. But the administration's decision to subject him, and itself, to the civilian justice system could pay big dividends. If he is eventually convicted, that will surely have greater credibility than would a conviction before a military court that heard some of the evidence in secret.