They may not be as well known as the FBI or as frequently the subject of myth and movies, but the men and women of the Bureau of Alcohol, Tobacco and Firearms deserve a lot of credit. They are the explosives experts of the federal law-enforcement agencies, and they have had primary responsibility for solving the abortion- clinic bombing cases. In a little more than two years, they have solved an impressive 20 of the 31 crimes; some of those convicted are already serving prison terms as long as 20 years.

Arrests made over the weekend involve eight bombings in Washington, Maryland and Virginia. The cases were solved not by tips, confessions or informers but by classic and difficult police work. A single fingerprint on a piece of tape used in a bomb and a hair on another were starting points. Bureau agents then traced all local sales of chemicals and carbon dioxide cylinders. They compared handwriting samples on receipts and worked with sales clerks to develop a composite drawing of one of the suspects. Little by little, through hard work and brainpower, they built a case based on physical evidence. Federal officials have maintained all along that there was no indication of a nationwide conspiracy in these cases, and the bureau director, Stephen Higgins, reiterated that position Saturday. Arrests and convictions obtained so far lead to the conclusion that small groups of fanatics in separate geographic areas are responsible for the bombings. Lawyers at the American Civil Liberties Union -- whose Washington office was bombed -- have expressed agreement with FBI Director William Webster, who refused, on the basis of the evidence, to take over the investigation of these cases and to elevate them to the status of a national conspiracy.

It is always wise to think twice before characterizing crimes as acts of a national terrorist operation, for broad inquiries in politically sensitive areas run the risk of going beyond the specific acts of violence that break the law. The fine work of the bureau in finding particular individuals and charging them with specific crimes is convincing evidence that good criminal investigators don't need broad theories and sweeping powers to find the bombers.