It's sometime in the future: terrorists have blown up a government building. In the rubble, an investigator searching with an ultraviolet light finds tiny particles that glow under its beam.

With a magnet he scoops up the fluorescing particles. Later they are analyzed and reported to Washington where their color-coded cores reveal the type of explosive involved, its manufacturer and the date and work shift on which it was made.

With this information, the explosive can be traced to retailers and, perhaps, to the person or persons who bought it.

This is the picture Treasury Department "taggants," the bureaucratic name for new, scientifically designed, inert particles made up of a seven-layered colored core, iron powder and outer layer of fluorescers that would respond to ultraviolet light.

These tiny identification taggants would be added to explosives -- making up less than one-one-thousandth of their volume. Although up to 90 percent of them would probably be destroyed in a detonation, those that remained could, when recovered and examined under a microscope, retain their color code and thus reveal their source.

After five years of research and testing, Treasury Department officials consider identification taggants technologically ready for inclusion in most commercially manufactured explosives.

A second, more exotic taggant -- to aid detection before detonation -- is still in development, though there are hopes it will be ready by late 1980.

Scientists are attempting to put a special inert vapor inside a tiny plastic capsule, smaller than the eye of a sewing needle. The vapor would seep through the capsule membrane to be detected by special equipment much like that already at airports.

The idea is to help locate explosives before they are set off, particularly at airports and buildings targeted by terrorists.

For two years Treasuty and other government agencies have been pushing for legislation to require that all explosives manufactured in the United States contain taggants. The latest version of the proposal is now before the House and Senate in an omnibus anti-terrorist bill.

Assistant Secretary of the Treasury Richard J. Davis recently said the program "will provide us with significant tools in the battle against terrorists and others who use explosives illegally."

Surpisingly, opposition has developed, from the National Rifle Association. Last year that group complained because taggants were going to be added to gun powder used in muzzleloaded guns and some home-loaded ammunition.

It was argued the taggants had not fully been tested for safety and could harm both sportsmen's gun barrels and the trajectory of their bullets.

Behind the objections, however, was NRA's fear that tagging explosive powders was a first step to federal gun control legislation.

A bill to authorize taggants but not in black and smokeless powders has been introduced this year by NRA supporters in Congress. Treasury opposes it because these powders are popular with terrorist bombers -- making up 20 percent of the criminal explosions.

The arrest rate for criminal bombings is among the lowest -- 8 percent -- of any crime category. One major block to investigators is that they often cannot determine what type of explosive caused the blast, and even when they can there is no way to narrow its source.

The explosive taggant program was born more than five years ago when a panel of government investigative agencies decided to turn to scientists for help.

The Aerospace Corp., under contract to Treasury's Bureau of Alcohol, Tobacco and Firearms, has acted as project coordinator.

Under its guidance an extensive research and testing program has been carried out by industrial giants. The 3M Co. has developed taggants, and the DuPont Corp., a leading manufacturer of explosives, has participated in testing programs.

A House Public Works subcommittee is expected to mark up the antiterrorist bill this week. But the legislative road is still long because two other House committees must act on the measure before it can go to the House floor for a vote.