Exercise Celestial Skónis: Part 1 – History, purpose, design and results of traditional forensic examinations of the 6th Collaborative materials exercise of the nuclear forensics International Technical Working Group

•The ITWG recently concluded its 6th and largest materials exercise, Celestial Skónis.•Participants of Celestial Skónis included 20 countries and the European Commission.•Participating labs were supported by 15 law enforcement agencies.•Participants examined evidence contaminated with Special Nuclear Material (SNM).•Participants used 36 nuclear forensic techniques to examine and analyze evidence.•Part 1 of this series summarizes results of traditional forensic examinations.•Part 2 of this series summarizes forensic analyses of the SNM that was included in CMX-6.•The ITWG works to improve best practices of nuclear forensics by facilitating exercises.