"Missing" lacks the streamlined tension of the dynamically paced, left-wing political thrillers -- "Z," "The Confession" and "State of Siege" -- that made Costa-Gavras' reputation a few years back. Nevertheless, it's an expertly acted and suggestive impression of battered American innocence and good will in the explosive, political environment of a South American country (obviously Chile, 1973) during a military coup.

Opening today at area theaters, "Missing" represents a provocative return to the genre of semi-documentary melodrama that became Costa-Gavras' stylistic specialty. The movie suggests that he went to considerable effort to modulate and complicate what was, indeed, a strictly kneejerk polemic in its original form, Thomas Hauser's 1978 nonfiction book "The Execution of Charles Horman." The screenplay by Costa-Gavras and Donald Stewart depicts the sequence of events reconstructed by Hauser and repeats his unproven accusation: that American officials, presumably located at the embassy in Santiago, must have approved, even "co-signed," a "kill order" which led to the execution of Horman, a personable, young free-lance journalist.

In a statement released Tuesday, the State Department defended its role in the case. It read in part, "The results of investigations . . . have found no evidence of involvement by any United States government personnel in the disappearance and death of Charles Horman."

The movie identifies closely with the point of view of Horman's father, Ed, portrayed by Jack Lemmon. An industrial designer from New York, the elder Horman flies to Santiago seeking information about his son's whereabouts. Some time after being picked up, Charles has vanished without a trace. In uneasy association with his daughter-in-law, Beth, played by Sissy Spacek, a young woman whose squalid manners and contemptuous attitude toward American officials offend him, Ed contacts a number of witnesses to Charles' comings and goings in the period leading up to the arrest and disappearance.

The upshot of this search, which incorporates a good deal of flashback reconstruction (some of it inserted or assembled in a patchwork fashion), is the discovery that Charles' body is among the corpses that have been accumulating during the weeks following the junta's seizure of power. The circumstances of his death remain obscure, but by this time a combination of grief and bitterness disposes Ed to endorse Beth's outlook and suspect representatives of his own government of deliberate hypocrisy as well as complicity in Charles' death.

While the filmmakers undoubtedly sympathize with the case as it is perceived by Ed and Beth (and outlined in Hauser's book), it's difficult to ignore the fact that the case remains unsolved and inconclusive. The accusatory thesis is there, but it doesn't necessarily follow that spectators will endorse it, even if they feel for Ed, as they almost certainly will, considering Lemmon's touching performance. The pathos operates quite apart from the argument. One can respond to Ed's sense of loss and futility without being persuaded by the editorial. Far from imposing a single interpretation on events, the movie tends to encourage ambivalence and emphasize the persistence of mysteries.

Costa-Gavras' style isn't as incisive as it used to be. "Missing" seems rhythmically slack compared to a movie like "Z," but, on the other hand, it accommodates social and emotional undercurrents that "Z" was perhaps too streamlined to incorporate. For example, the setting smolders with menacing possibilities that occasionally ignite, but it's the feel of violence in the atmosphere that gets to you more than any particular manifestation. It's really oppressive. In this setting, where thousands of armed young men in an insurrectionary lather are capable of using force, the possibilities of random, arbitrary violence seem to expand infinitely. Maybe Horman's death was "authorized," but it's easy to imagine unauthorized killings abounding.

There are also a number of other complications. The generational hostility between Ed and Beth is very convincing and carries a strong implication that father and son had drifted apart in a way that is especially painful for the survivor, since the gulf can never be bridged. Young Americans like Charles and Beth and their expatriate friends also carry suggestions of radical-chic fecklessness (and even nastiness in the case of abrasive, unlovable Beth). Attracted to Chile under Allende and by the prospect of adventure or social utopianism, they possess an innocence only superficially different from Ed's. John Shea's intriguing impersonation of Charles suggests a charming, watchful, clever young man whose curiosity might lead him into danger. Even the American Foreign Service officers accused of engineering or condoning Charles' death -- the key performers, a first-rate bunch, are Charles Cioffi, David Clennon, Richard Venture, Jerry Hardin and Richard Bradford -- get the opportunity to defend themselves and make some telling, cold-eyed debating points about the way the world works. They aren't required to shrivel up with shame when Ed condemns them.

As a result, the context in which the story of Charles Horman's disappearance and death unfolds becomes unusually dense and complex. On the surface "Missing" may resemble a polemical exercise, but just beneath the surface there's a fascinating network of turbulent emotions, clashing values, and unanswered questions.