A mysterious bomb-making experiment that ended with the accidental death of a government scientist has remained an official secret for more than five years, leaving his family in the dark about what went wrong.

Terry Jupp, a scientist with the Ministry of Defence, was engulfed in flames during a joint Anglo-American counter-terrorism project intended to discover more about al-Qaida's bomb-making capacities.

There has been no inquest into his death, as the coroner has been waiting for the MoD to disclose information about the incident. An attempt to prosecute the scientist's manager for manslaughter ended when prosecutors said they were withdrawing the charge, but said the case was too "sensitive" to explain that decision in open court.

The Guardian has established that Jupp was a member of a small team of British and US scientists making bombs from ingredients of the sort that terrorists could obtain. There is also evidence pointing to experiments to discover more about radiological dispersal devices - so-called dirty bombs - which use conventional explosives to scatter radioactive material.

But such a project would have been controversial as the open-air experiment that ended in Jupp's death was conducted at a weapons testing centre on an island in the Thames estuary 10 miles from Southend, Essex.

Meanwhile, the scientist's family despair of discovering what happened. "I feel these people high up want it swept under the carpet," said Jupp's mother Anne. "The death of one man is nothing to upset them too much, I suppose. But it does upset us."

Jupp was 46, married with two children, and had been with the MoD for almost 25 years. At the time of the accident he was working with the Forensic Explosive Laboratory, a division of the ministry's Defence Science and Technology Laboratory (Dstl).

On August 14 2002, he and his team was conducting a series of highly classified experiments on Foulness, a remote island that is part of MoD's vast weapons testing centre at Shoeburyness, east of Southend.

Blending several readily-available ingredients, then pouring the mix into old paint tins, they built a number of 10kg bombs. Sources familiar with the case say the fatal experiment involved mixing three over-the-counter ingredients including ammonium nitrate fertiliser and a powdered metal.

Jupp was asked to prime the mix with a small amount of high explosive, but for reasons that remain unclear it ignited spontaneously. Jupp was consumed by a fireball and suffered 80% burns, dying six days later.

Court case

An investigation by the Health and Safety Executive and MoD police resulted in two of Jupp's managers being charged with manslaughter and being brought before the Old Bailey in April 2005.

The charge against one man was thrown out when the judge ruled there was insufficient evidence. The second man denied the charge and the case against him dragged on for years, before being abandoned after a review involving Lord Goldsmith, then attorney general.

Gareth Patterson, prosecuting, told the Old Bailey in March last year that information had emerged from subsequent experiments, but added: "The difficulties of the sensitivities of this case are such that I cannot go into too much detail about the information in open court."

Crown Prosecution Service sources said the case was hampered because one of the American scientists refused to testify, while other officials said there was concern in both countries that a trial could expose the nature of the experiment.

According to a number of officials in Britain and the US, the Dstl had carried out a series of secret experiments with the US national laboratory in New Mexico to find out more about the sort of bombs terrorists could build.

One of the Old Bailey defendants was the key figure on the British side, these officials say.

According to these sources, in August 2002, less than a year after the September 11 attacks British and American scientists were anxious to establish whether al-Qaida could build a dirty bomb using conventional explosives surrounded by radioactive material.

"They were looking into the most likely explosives to be used to scatter radiation," said one. "They wanted to know how big such a bomb might be and how far it would scatter the radiation. They were experimenting with chemicals available over the counter to see how powerful an explosion could be produced."

It is unclear whether the bomb that killed Jupp contained radioactive material, and the MoD refuses to say whether he was involved in a dirty bomb project.

Asked whether it has carried out such experiments at Shoeburyness, the MoD would say only: "The Dstl is involved in classified work that is of national importance, protecting UK armed forces and the public from very real threats."

What is clear is that Shoeburyness has hosted some highly unusual activities involving radioactive material.

According to an Environment Agency report, at the time of the accident it was the scene of "a major programme of nuclear warhead decommissioning". Between 1998 and 2003, the report said, high explosive extracted from free-fall nuclear bombs and Polaris missile warheads, which had been contaminated with tritium and uranium, was taken to Shoeburyness for disposal.

This was achieved by taking the high explosive to a remote corner of Foulness island, and by simply blowing it up.

The agency said these operations posed no risk to human health, as the level of radioactive contamination was low. But the footpath skirting the bleak coastline south of the site is lined with signs warning the public not to fish there and to never take away shellfish.

Jupp's family knew nothing about his work and have been told nothing about the experiment that led to his death.

His father Roy said: "He said he worked in plastics. That was the only thing he ever told us."

Jupp's sister Alison Davis added: "We were absolutely stunned when the phone call came though to tell us about an explosion. We thought: 'Why would Terry be involved in an explosion?'"

Delays in the criminal case - which they had hoped would shed light on the tragedy - were a cause of immense frustration. Now they have no idea when an inquest may be held.

The case was handed over to the local coroner in Essex last March, but it took the MoD 12 months to hand over correspondence relating to the case.

The MoD said this was down to "technical things" but would not elaborate.

A spokesman said that some of the documentation about the death of Terry Jupp remained at the ministry, and that while the coroner will be allowed to view it, "he will not be allowed to take it away".