ON THE TRAIL OF THE TERRORISTS

By MICHAEL WINES; MICHAEL WINES IS A REPORTER IN THE WASHINGTON BUREAU OF THE NEW YORK TIMES.

THE FALL OF PAN AM 103
Inside the Lockerbie Investigation.
By Steven Emerson and Brian Duffy.
304 pp. New York: G. P. Putnam's Sons. $21.95.

''Terrorists haven't had to deal with their acts so much in the context of law enforcement,'' an American explosives expert said in an interview four months after a carefully concealed bomb blew Pan American World Airways Flight 103 out of Scotland's nighttime sky. ''They consider it a success when the bomb goes off. What they don't understand is that that's when their problems start. The system takes over, and law enforcement does its investigation, and sooner or later the terrorist makes a mistake. And then we've got him.''

Awed by the acts of nightmarish figures like Abu Nidal and Ahmed Jabril, the public sometimes forgets that blowing up airliners is first a crime and only second a political spectacle. The authors of ''The Fall of Pan Am 103'' have no such illusions. Steven Emerson, a former senior editor at U.S. News & World Report, and Brian Duffy, an assistant managing editor at that magazine, follow the inquiry into the 1988 bombing of Pan Am Flight 103 like detectives, trailing American and Scottish investigators as they chase clues around the world, bloodhound-style, to a logical conclusion.

The authors also offer intriguing tales of bungling and cover-up by the third partner in the Pan Am inquiry, West Germany, which had the leading suspects in captivity and allowed them to slip away - whether by design or incompetence is not entirely certain.

Since the manhunt has been cloaked in secrecy, the authors had no easy task. But with the apparent assistance of two key officials, Mr. Emerson and Mr. Duffy have put together a surpassing account of the investigation to date, rich with drama and studded with the sort of anecdotal details that give the story the appearance of depth and weight.

The Pan Am inquiry is a detective story for the ages - a relentlessly effective mix of 21st-century physics and 19th-century shoe leather. Aided by spy-satellite photographs, investigators combed 845 square miles of Scottish countryside near Lockerbie, collecting shattered fragments of the aircraft and its cargo. From those scraps, they rebuilt the plane and a metal baggage container that the explosion had ripped apart. Chemists identified the suitcase that had held the bomb, then the scorched clothes that had been packed around it. Finally, the police traced the clothes to the very shop where they had been sold, two months before the disaster, and even prepared a sketch of the Middle Eastern man who had bought them.

It is in describing the technical and human nitty-gritty of an international manhunt that ''The Fall of Pan Am 103'' excels - how the search for debris was so thorough that it turned up a hearing aid that an American official had lost in a Scottish bog or how the first evidence of a bomb blast aboard the plane was discovered and confirmed. The doggedness of these sleuths as portrayed by Mr. Emerson and Mr. Duffy is mind-boggling. Their forensic wizardry would leave even Sherlock Holmes shaking his head in wonder.

The narrative occasionally grates as the authors betray their newsmagazine heritage by making too many paragraphs into long drum rolls leading to a five-or six-word clang of cymbals: ''Sadly, it would get worse'' or ''So much for the London theory.'' And perhaps it is because of the abundance of newsmagazine-style color that the absence of detail surrounding a few of the book's most newsworthy disclosures stands out so sharply. On one hand, the authors admirably flesh out reports of West German misfeasance in the inquiry, relating story after story of bungled police raids and overlooked airplane bombs that raise real questions about Bonn's obstruction of the decade's biggest criminal investigation.

On the other hand, the authors raise equally serious charges of Iranian complicity in the bombing. But instead of weaving these revelations into the day-to-day story of the investigation, they drop them almost casually at the end, without much substantiation. Mr. Emerson and Mr. Duffy are not to be lightly dismissed; they are respected journalists who talked to 250 people, including senior law enforcement and intelligence officials in seven nations. Still, one longs for more evidence on such questions as how it was ascertained that Iran provided plastic explosives for the bombing in a diplomatic pouch - evidence that, if in hand, would seem to demand an American response.

One also finishes the book wishing for some diplomatic and political reporting to supplement the police-blotter details. The authors provide a thumbnail sketch of Middle East terrorism, but not much on how American policy copes with it. They build a convincing circumstantial case against Iran and its terrorist agents, but they do not tell us what the Bush Administration thinks of that case or what it intends to do about it, if anything.

All that may be beyond the intent of a book that purports to do nothing more or less than tell the Pan Am detective story, and the authors surely tell it better than anyone else. As interesting as this tale of high-tech sleuthing is, however, one would like to know if all the time and toil invested by the Scots and the Americans will go for naught.