The possibility of accidental contamination of a suspect by gunshot residues (GSRs) is considered. If two hypotheses are taken into account ("the suspect has shot a firearm" and "the suspect has not shot a firearm"), the likelihood ratio of the conditional probabilities of finding a number n of GSRs is defined. Choosing two Poisson distributions, the parameter lambda of the first one coincides with the mean number of GSRs that can be found on a firearm shooter, while the parameter mu of the second one is the mean number of GSRs that can be found on a nonshooter. In this scenario, the likelihood ratio of the conditional probabilities of finding a number n of GSRs in the two hypotheses can be easily calculated. The evaluation of the two parameters lambda and mu and of the goodness of the two probability distributions is performed by using different sets of data: "exclusive" lead-antimony-barium GSRs have been detected in two populations of 31 and 28 police officers at diverse fixed times since firearm practice, and in a population of 81 police officers who stated that they had not handled firearms for almost 1 month. The results show that the Poisson distributions well fit the data for both shooters and nonshooters, and that the probability of detection of two or more GSRs is normally greater if the suspect has shot firearms.

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The possibility of detection of lead-antimony-barium aggregates from non-firearm sources is confirmed according to the tests performed on brake pads, and firework and automobile workers. Moreover, information on particles taken from cartridge cases shows the relative feeble importance of the morphology in distinguishing gunshot residues. Furthermore, also the presence in the spectrum of other elements (e.g. iron) is not so conclusive.In this panorama, the possibility of discriminating gunshot residue particles from other non-firearm lead-antimony-barium aggregates is investigated: the proposed method is based on X-ray mapping technique-currently applied and used in Reparto Carabinieri Investigazioni Scientifiche in Rome, the forensic service of Italian Carabinieri-according to which the spatial distribution of the emission energy of each element of the sample is pictured. Gunshot residues present the same lead-antimonybarium distribution (or at least the same antimony-barium distribution with lead nodules), as some other environmental occupational aggregates do not (different plaques of lead, antimony and barium).So, X-ray mapping technique can offer a new fundamental evaluation parameter in analysis of gunshot residues with scanning electron microscopy/energy-dispersive spectrometry, and new standards could be considered. # 2004 Published by Elsevier Ireland Ltd.

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