Semtex error could free 12 IRA men

The criminal justice system was dealt another blow last night when the Government revealed that at least 12 IRA prisoners - one-third of all those jailed in the last six years - may have been wrongly convicted because of contamination in a forensic science laboratory.

Government sources stressed that the chances of there being any miscarriages of justice were slight, but conceded the number of cases involved could be even higher following an independent investigation into the work at government science laboratories in Kent.

The affair is a major embarrassment for the Government, which had been hoping to put behind it the spectre of a series of miscarriages of justice involving innocent Irish people - like the Birmingham Six, the Guildford Four, the Maguire family and Judith Ward. All involved flawed scientific evidence. It is also a setback to the fight against terrorism because of police fears that guilty men could go free. Last night Sinn Fein advised all republican prisoners jailed in England on explosives charges to challenge their convictions.

Neither the scientists, the Home Office, nor the Crown Prosecution Service were prepared to identify which cases had been called into question by the discovery two months ago of explosives traces on a key piece of second-hand laboratory equipment. But 38 people have been convicted of explosives charges since 1989 - the time from which the Home Office admits contamination may have occurred - and high-profile IRA cases now under suspicion are believed to include John Kinsella, convicted of the Warrington gas works bombing, and Feilim O'Hadhmaill, a university lecturer convicted of possessing Semtex.

The news of the accidental discovery, two months ago, of contamination by RDX, a component of Semtex - favoured by the IRA - was broken to the Commons yesterday. Traces were discovered by accident in a crucial piece of machinery used in the analysis of evidence in bomb cases.

The equipment, a centrifuge machine the size of a microwave oven, has been used in almost all forensic tests on terrorist bombings since 1989. But it emerged yesterday that neither the machine, which spins the dirt out of samples for testing, nor its parts had been tested or changed between experiments - despite what are supposed to be routine weekly contamination checks at the laboratory.

The news provoked a furious political row, with Labour demanding that Michael Howard, the Home Secretary, make a full emergency statement to the House today.

Jack Straw, the shadow Home Secretary, said: "This is a very serious matter and is certainly one in which the Home Secretary should have made a full statement to the House of Commons instead of sheltering behind an incomprehensible written answer."

Irish government sources said: "The possibility that any person might be convicted on the basis of contaminated evidence is obviously a cause for concern. In view of the potential gravity of the issue we trust that the review and subsequent referral to the Court of Appeal will proceed in an expeditious manner."

Mr Howard maintained last night that he doubted the inquiry would lead to the freeing of any IRA prisoners. "The chances of there having been a miscarriage of justice as a result of what has been discovered are very small, but I don't want to take any risks or leave any stone unturned. That is why we are having an independent investigation to look at this thoroughly, rigorously and speedily," he said.

Last night, lawyers called for the inquiry to go back to 1974 because of suggestions that contamination at the same laboratory played a part in the wrongful conviction of both the Maguire family and Judith Ward. The lab was run initially by the Ministry of Defence and more recently by the Defence Evaluation and Res earch Agency. In fact, it was Professor Brian Caddy, of Strathclyde University - called in to investigate the 500 cases which have passed through the laboratory since 1989 - who concluded that laboratory contamination of the Maguires' hand swabs was the only explanati on for the unique pattern of positive results. Last night Alastair Logan, solicitor for the Maguire family, said: "This simply demonstrates that this laboratory has never been able to control contamination - the Home Secretary should widen the inquiry to cover the period from 1973." Mr Howard accepted in his Commons written answer that the discovery could lead to some terrorist cases being referred to the Court of Appeal. He said: "There is a small theoretical possibility that any casework sample showing RDX traces may have been aff ected by the centrifuge contamination. On present information, there may be around a dozen such cases which resulted in convictions." Those cases - and all the others - will now be examined by Professor Caddy, who will decide whether the centrifuge contaminated key evidence, the extent to which forensic evidence helped to secure a conviction, and the procedures used in analysis work at the laboratory. tThe Forensic Explosives Laboratory at Fort Halstead in Sevenoaks, Kent, has one of the most sensitive detection systems in the world. In prosecuting an alleged terrorist, the presence of even the tiniest amount of RDX (a component of Semtex) or PETN (us ed in detonators) would be presented as proof positive of the accused's involvement with explosives, writes Charles Arthur. But on 14 March, an accident in the laboratory revealed that one of the key pieces of equipment - a centrifuge - was contaminated with 30 millionths of a gram of RDX. Furthermore, this could in turn have contaminated some of the case samples. The samples to be tested arrive in the form of cotton swabs from a crime scene. Their preparation for final analysis by gas chromatograph (which identifies chemicals) would often use the centrifuge for purification. The contamination found yesterdaywas in one of eight rubber bungs which pad the test-tubes holding the samples. The report released yesterday says that the bung had enough RDX to contaminate many samples. Th key question is whether any tests were falsely contaminated. The forensic tests always included a "control" sample known not to contain any explosives. But Dr Marshall, the FEL's head, admits that if a control sample tested positive, that test would be abandoned. This means that only the tests which were obviously wrong were rejected. But it is not yet known how many control samples tested negative, while the case sample wrongly tested positive.