Deadly power of the explosive used in Oklahoma and Bali

Ammonium nitrate fertiliser has become established as one of the most deadly components of al Qaeda's campaign of international terrorism.

Massive car bombs containing the chemical have left a trail of death and destruction around the world from Bali to the US. It was first used in the partially-aborted 1993 attack on the World Trade Center in New York, then in the huge bomb found outside the US embassy in Karachi, Pakistan in June 2002.

But it was in Bali in October 2002 that al Qaeda used fertiliser bombs to their most devastating effect when 202 tourists, including 26 Britons, died.

Ammonium nitrate has also been used in several much earlier IRA attacks. The 1995 truck bomb detonated by Timothy McVeigh that reduced the Oklahoma City federal building to rubble - killing 168 people - contained more than two tons of the fertiliser. Soho bomber David Copeland was also found to have two kilos of the chemical in his room in 1999.

Until now it has been relatively easy for al Qaeda bombers and other terrorists to use the chemical. Most countries, including the US and Australia, do not regulate the sale of fertiliser-grade ammonium nitrate, but in the EU it is already restricted. David Heather of the Agricultural Industries Confederation in Peterborough said: "We are all aware of the sensitivities."

Vigilance and intelligence are the best weapons against terrorism. But it may be possible to make it harder for terrorists to turn a readilyavailable chemical into bombs.

Anything sold as fertiliser must pass a detonation resistance. In the EU, fertiliser-grade ammonium nitrate is manufactured to higher standards than the explosive grade, with large, dense granules to prevent them absorbing fuel oil. Stabilisers are sometimes added to prevent the granules breaking down.

Another approach is to mark fertilisers so that any bomb can at least be tracked back to its source.

Authentix of Dallas, Texas, says it can chemically tag any fertiliser during manufacture. In the case of ammonium nitrate, molecules that contain different isotopes of nitrogen and hydrogen can be added in concentrations of parts per billion. Millions of tons of ammonium nitrate are produced each year for use as a fertiliser.

Mining companies turn small quantities into an explosive by mixing the chemical with fuel oil. While it is not necessarily easy for would-be bombers to do this with fertiliser-grade ammonium nitrate, it is not impossible. Now Speciality Fertilizer Products, a company based in Belton, Missouri, is patenting a water-soluble polymer coating for the fertiliser granules that repels fuel oil.

The coating dissolves in soil, so it would not interfere with ammonium nitrate's main function as fertiliser. If it works and is widely adopted, it could make it harder for terrorists to turn fertiliser-grade ammonium nitrate into bombs, and could also help prevent industrial accidents.