ATTORNEY GENERAL Benjamin R. Civiletti testified before the Senate committee investigating the Billy Carter case the same way he runs the Department of Justice -- quietly and precisely. He knew exactly what he wanted to tell the committee and did not stray beyond the bounds of his prepared statement -- not even in response to provocative questions. As a result, he presented the department's version of how it handled this political hot potato in the most favorable light.

But it is that particular lawyerly quality that has gotten Mr. Civiletti into trouble before. His most glaring mistake in the Billy Carter affair was to deny at a news conference that he had talked about the case with President Carter.He had to take his words back the next day when it became clear that others did not make the distinction he did between a thorough discussion (which he did not have with the president) and a brief conversation (which he did).

The attorney general's habit of dealing in such fine distinctions and providing no more information in response to a question than is absolutely required to answer its exact terms makes us leery of accepting his testimony as the final word on all that went on at the Justice Department. Certainly, he has made it appear that he had little to do with the Billy Carter case and that his subordinates handled it in a routine, professional way.

It does seem clear that Mr. Civilett's conversation with the president had no impact on the way those subordinates treated the case. It ended up about the way it should have, with Billy Carter's registration as a foreign agent and the spreading on the public record of his financial arrangement with the Libyan government. Whether Mr. Civiletti acted improperly in telling the president anything about the case will remain a matter of controversy. Two senators told him they had doubts about the propriety of that conversation, while two others -- with whom we tend to agree -- said he waited too long before telling the president what was going on.

Mr. Civiletti's other intervention in the Billy Carter case raises a second serious question. In late March or early April, he ordered the investigation kept open because he knew something his staff did not know. Should he have given his investigators the highly classified intelligence data he had received, or was he right to make them proceed with the investigation in the dark? That's a close question, given the sensitive nature of the information and, in one sense, an academic one, since the investigators got the same data elsewhere.

Much more important, it seems to us, is what national security adviser Zbigniew Brzezinski did with similar highly classified information he had received somewhat earlier. If Mr. Brzezinski passed it along to Billy Carter, more than a simple act of giving friendly advice to the president's brother may be involved. It is to this aspect of the matter that the Senate investigation turns this week. It now seems that if any wrongdoing at all is to be discovered, it will lie in the eagerness of some members of the White House staff to help out Billy Carter.