In October, huge numbers of crabs, as well as some lobsters, were reported to have washed up on beaches in Teesside, north-west England. Some were dead and some alive, and a number found among piles of seaweed.
The EA said it had screened samples of water, sediment and crab for more than 1,000 potential chemical contaminants but found no anomalies that could lead to the deaths.
It is now working together with the Centre for Environment, Fisheries and Aquaculture Science (Cefas), North Eastern Inshore Fisheries and Conservation Authority (NEIFCA), the Marine Management Organisation (MMO), Food Standards Agency, UK Health Security Agency (UKHSA) and local authorities to establish the cause of the deaths.
Earlier this month, the EA ruled out sewage pollution, along with seismic activity and underwater cables.
The regulator said the investigation would now focus on whether disease or a natural event could have been responsible.
Sarah Jennings, operations manager for the EA, said: “Our environment officers have also reviewed environmental permits and scrutinised industrial sites in the Teesside area, but again found no evidence of abnormal discharges that could lead to an event of this scale.
“In a bid to better understand the scale of the incident, our survey vessel the Humber Guardian has taken samples from the seabed, which show that only crabs and lobsters appear to be affected.
“By combining this evidence we have ruled out chemical pollution and sewage as likely causes, and the investigation will now focus more on whether disease or a natural event could have been responsible for the deaths.”