HOMEMADE bombs are made of such common materials that nations find it hard to stop the terrorists who use them. Restrict one compound and terrorists move onto another. All they need to make a bomb explode is oxygen, fuel, and heat.
But there are steps the United States can take to make it harder for terrorists to get some of these elements. As a part of its $1.5 billion antiterrorism package, the Clinton administration is proposing, among other things, using so-called ''taggants'' to trace bombmaking material.
Yet many experts believe there may be an even more obvious way to use technology to stop bombers: Restrict access to detonators. It may also prove less politically sensitive. In the past, the powerful National Rifle Association and other right-to-bear-arms groups have opposed attempts to require bomb-tracing technologies.
The homemade bomb that destroyed the federal building in Oklahoma City April 19 was a type of device well-known to bomb experts worldwide. Investigators believe the terrorists used ANFO -- a mixture of ammonium nitrate and fuel oil. ANFO bombs have been used in Northern Ireland and Egypt. An ANFO bomb claimed the lives of 241 US Marines in Lebanon in 1983.
Terrorists choose ANFO because the technology is simple. The main ingredients are easy to get. Diesel fuel is as close as the corner filling station. Ammonium nitrate is a common fertilizer used on wheat and pasture land. Farmers buy it by the ton.
These ingredients are also relatively cheap. A senior federal official said recently that the Oklahoma City bomb -- which agents believe was made up of some 20 to 25 plastic drums of ANFO -- cost about $5,000 to make.
ANFO is easy to transport. The crystals of the ammonium nitrate absorb the fuel and won't explode without tremendous heat.
Detonators hard to get
The trick to making ANFO bombs work is a good detonator that will produce the necessary heat. ''They pretty much have to use high explosives,'' says Maurice Greiner, a consultant to the chemical industry on ammonium nitrate fertilizer, based in Seattle.
Investigators think the bomb used detonating cord. Explosive experts say such a cord, rolled up in a ball, could produce enough heat to set off the ANFO. This detonator -- the heat trigger -- is the most difficult ingredient for terrorists to get.
Federal laws restrict access to detonating wire and other high-explosive items. But federal law doesn't cover all high-explosive sales, says Cindy Douglass, executive vice president of the Institute of Makers of Explosives (IME), a trade group here. And state laws are inconsistent.
For example, the federal Bureau of Alcohol, Tobacco and Firearms (BATF) requires a permit before someone can buy explosives that will cross state lines, Ms. Douglass says. The permit involves an identification card and a background check.
But sales of explosives used within a state fall under state law. Some states require permits. Others, such as Kansas, only ask the buyer to fill out a registration form and show identification, Douglass says. There's little to stop a would-be terrorist from buying high explosives in a more permissive state and using them somewhere else, she adds.
The institute has long supported measures requiring federal oversight of all high-explosives purchases. Mr. Greiner is also an advocate of tightening restrictions on the items already most difficult to get. ''Make it so hard for people to get the trigger'' that it doesn't fall into terrorists' hands, he says.
Nevertheless, Senate hearings here late last week suggest that lawmakers and law-enforcement officials are looking at other technological fixes. One involves adding tiny chips or chemical tags to potential explosives. If bomb experts found traces of the tags, they could determine the kind of explosives used and, perhaps, who bought them.
IME supports chemical tags, used to detect the kind of explosive. But it has balked, along with the National Rifle Association, at adding the chips that could identify the specific manufacturer, saying such tags have been found to make some explosives unstable.
In 1976, BATF began experimenting with the chips, tagging some 7 million pounds of commercial explosives. The chips even helped convict a man in Maryland who used dynamite to kill his nephew. BATF claims the technology is safe.
But tags would probably not be added to potential explosives, such as fertilizers, because the volumes involved are too big.
''It would be hard to tag such common materials,'' says James Rancourt, president of Polymer Solutions, a Blacksburg, Va., chemical-analysis company. ''When you see someone mixing fertilizer with diesel fuel, how much more common can you get?''
The other proposal lawmakers are examining is adding limestone to ammonium nitrate fertilizer -- thereby lessening the fertilizer's bombmaking potential. A few countries, including South Africa and Britain, already do this. But British restrictions have not stopped terrorists in Northern Ireland from moving to other common materials to make their bombs.
Fuel, the third element in detonating a bomb, is so commonplace that no one is advocating restricting its use.