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.

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.