Hong Kong's only independent forensic expert yesterday criticised Government scientific work purportedly showing a defendant had traces of high explosives under his fingernails. In the Court of First Instance trial of three alleged henchmen of 'Big Spender' Cheung Tsz-keung, expert witness Dr Sheilah Hamilton said critical forensic tests where 820kg of explosives were found were worthless. Dr Hamilton said 'control' samples should have been taken from the shoes and bodies of police officers when they arrested two men for allegedly possessing a potentially catastrophic mix of ammonium nitrate, TNT, fuel oil and wood chips. Government forensic officers instead went to an address that was 30 minutes' drive from the alleged crime scene to take samples - and did so almost a year after the explosives were seized. 'It's nearly a year later . . . and more to the point, it wasn't the same part of the New Territories. [The result] doesn't help in the slightest,' she said. 'It's academic at best.' Chan Sum-yau, 47, Wong Fung-kei, 44, and Lau Cheuk-fan, 32, have each pleaded not guilty to possessing high explosives with intent to cause severe damage to life and property. The court had already heard the haul was big enough to blow up any building in Hong Kong and cause carnage over 100 metres. Prosecutors claim that traces of ammonium nitrate found on two of the three men prove they handled the explosives. Lawyers for the trio argue such traces could equally have come from ammonium nitrate fertiliser which is the commonest kind used in Hong Kong. The traces found are key evidence in the prosecution's case. Dr Hamilton - a former top government scientist with 30 years' experience - said in reply to Chan's lawyer, Peter Lavac, that ammonium nitrate traces found under the defendant's fingernails could 'certainly' have come from fertiliser. Dr Hamilton now runs a private consultancy which offers the only independent forensic expertise in the SAR. The case continues before Mr Justice Peter Nguyen.