IS THE INVESTIGATION of Chilean political exile Orlando Letelier's assassination moving as expeditiously as it should? In the nearly seven months since the former Allende government official and an American colleague were killed in the explosion of their car in Sheridan Circle, no one has been arrested or indicted. This in itself is inconclusive, given the nature of the case. Yet the question is unavoidable: Is there a political brake on the investigation, one produced not perhaps by any calculated policy of the Carter administration but by the resistance of certain parts of the bureaucracy?

Consider, for instance, the continuing leaks of materials supposedly contained in Mr. Letelier's briefcase, which was found at the scene of the explosion. The contents were duly examined by the authorities and returned to Mr. Letelier's associates. Then the leaks began. They portray Mr. Letelier as an exile doing political battle against the Pinochet dictatorship - precisely what he claimed to be. But they also reflect an intent to discredit him as a leftist, a manipulator of gullible Americans and a Cuban agent. The disturbing thing is that the leaks appear to be coming from some of the very quarters charged with investigating the crime. If this is so, one cannot avoid asking whether these officials are more interested in rationalizing a coverup or doing their proper job. To the investigators, Mr. Letelier's politics should be irrelevant. Would it be asking too much of the FBI and the Metropolitan Police to say so?

Yet another suspicion has intruded into this case. It is that the investigation is being impeded by the CIA, either on account of its own connections with Cuban exiles who may have committed the crime, or on account of its connections with its Chilean counterpart, DINA. The CIA "categorically" denies the first suggestion and pronounces the second "nonsense." It states that it is providing the FBI all available pertinent information. But the background condition of international intrigue that gives rise to such suspicions remains.

Who killed Orlando Letelier? It is reported that investigators are about to question a former CIA explosives experts who allegedly once tried to hire Cuban exiles to kill a political foe of Libya's Colonel Qaddafi. Certain circumstances suggest that he may have been hired for a similar mission by the Chilean government. We have no idea whether this latest lead is a promising one. Nor evidently do the authorities. In the absence of an early break, however, it seems to us that the authorities have a special reason to dispel the political uncertainties that have arisen over the Letelier investigation by offering as full an accounting as they can, without compromising their investigation, of how it is proceeding so far. This is no ordinary case. It cannot be handled in an ordinary way.