Three Americans are due to be sentenced next month for their involvement in a plot to explode a cyanide bomb capable of killing thousands of people, in a case that has served as a reminder that homegrown terrorism is still a menace in a country permanently braced for another attack from abroad.

It is still unclear for what the bomb and the arsenal of other weapons unearthed in the small town of Noonday, Texas, would have been used.

The conspirators - rightwing extremists who were caught with forged identity passes to the United Nations and the Pentagon, and a variety of racist and anti-government pamphlets - have refused to cooperate with investigators, who believe others involved in the plot may still be at large.

The central figure in the case, William Krar, is a small-scale manufacturer of gun components who has pleaded guilty to possessing a chemical weapon and faces a possible life sentence.

The plot was uncovered by accident in early 2002 when Krar and his partner, Judith Bruey, posted a package to a third conspirator, Edward Feltus, a member of a rightwing group called the New Jersey militia. The package, filled with fake identity documents and a note saying "we would hate to have this fall into the wrong hands" was delivered to the wrong person.

When investigators searched a storeroom rented by Krar and Bruey in Noonday, 100 miles east of Dallas, they found half a million rounds of ammunition, 65 pipebombs and briefcases that could be detonated by remote control, as well as 800g of almost pure sodium cyanide .

According to the Los Angeles Times, the cyanide was already packed in an ammunition canister, next to a variety of acids and bombmaking formulas.

Investigators believe that such a bomb would send up a cloud of poison that could kill everyone inside a large building.

"It was clearly one of the most lethal arsenals associated with the US paramilitary right in the past 20 years," said Daniel Levitas, the author of a book on rightwing extremism, The Terrorist Next Door: The Militia Movement and the Radical Right.

It is, however, far from an isolated incident. Mark Potok, who keeps tabs on hate groups at the Southern Poverty Law Centre in Alabama, says up to 40 major conspiracies involving domestic terrorism have been uncovered since the 1995 Oklahoma attack by a rightwing war veteran, Timothy McVeigh, which killed 168 people.

One foiled plot, by Ku Klux Klan members in 1997, was aimed at blowing up a Texan oil refinery and could have killed up to 30,000 people in the immediate vicinity.

But domestic conspiracies have received much less publicity than foreign threats.

"There is no question at all that had William Krar been a Muslim, this would have been announced from the steps of the justice department," Mr Potok said. The arrests were announced locally in Texas but received hardly any press coverage.

Mr Levitas said Mr Krar had been arrested on previous occasions on gun charges but then released.

"He came on to the radar screen of federal authorities and promptly fell off their radar. It points to a failing of the justice department to exercise due diligence."

A series of anthrax letter attacks which took place in the weeks after September 11 2001 are also thought by FBI investigators to have been the work of an American fanatic. The case has not yet been solved and has similarly been largely forgotten.

The justice department has denied taking domestic terrorism less seriously than foreign threats, saying it pursued all violations vigorously.