Data and written reports detailing the evidence used to inform the Environment Agency and Defra-group investigation into Crustacean Mortality events of October 2021. Data from the Environment Agency comprise measurements of ~1500 chemical pollutants in sediment, water and biota (Crustacea spp) samples using a Gas Chromatography Mass Spectrometry (GC/MS) screening technique. Reports from Cefas comprise specific reports detailing a) statutory reporting of disease screening by the Fish Health Inspectorate, b) assessment of parasites in Hommarus gammarus, c) opportune microtoxicity testing of sediment samples collected from disposal sites in the Tees Bay in 2021, and d) all other components of the investigation that Cefas were involved in, including the modelling of sediment disposal, the modelling of chemical release of pyridine, and the assessment of evidence for a harmful algal bloom.