The aim of this research was to modify the high-resolution gamma spectrometry method for faster and more precise determination of activity from complex spectra of nuclear and other radioactive materials, i.e. for use in nuclear forensics. The researched materials are divided into five groups and analysed by high-resolution gamma-spectrometry, and for some samples using additional methods. The analysis of materials from the location of the remediation of the radiological extraordinary event indicated radioactive contamination with 137Cs. Analysis of radioactive material in the barrels indicated the presence of radionuclides 226Ra, 60Co, 137Cs, 238U, 241Am and 125Sb, and confirmed the uneven distribution of radionuclides in the concrete matrices inside the barrels. In test tubes of unknown composition, 235U and 238U were detected in the natural isotopic ratio. All applied methods confirmed the inhomogeneous composition in powder samples of unknown origin. Two samples contain radionuclides from the 232Th chain and have a similar composition, while the third sample is more radioactive, contains radionuclides from the 238U chain and the mineral urophane. In samples of powder and jewellery from the Karasovići case uranium was detected. In part of the jewellery, the natural isotopic ratio was disturbed in favour of 238U. The powder contains uranium-rich phases masuyite, uraninite and betafite, which do not originate from Croatia. The high-resolution gamma spectrometry method has proven to be a reliable primary method for use in nuclear forensics and sample analysis procedures have been adapted for the analysis of nuclear and other radioactive material.