The Mail told last month how hundreds of crabs had been reported dead on beaches at Seaton Carew.
An official investigation, which included input from the Environment Agency and Centre for Environment, Fisheries and Aquaculture Science (Cefas), was launched with tests soon discounting sewage and seismic activity as causes.
Now the multi-agency inquiry has also eliminated chemical pollution as a likely trigger.
Sarah Jennings, operations manager from the Environment Agency, said: "We understand how distressing this incident is for the local shellfish industry and for members of the public so this investigation has been a top priority for Environment Agency and Cefas laboratories.
"We've used both traditional and innovative screening methods to analyse samples of water, sediment and crab looking for traces of contamination. We’ve screened for over 1,000 potential chemical contaminants but found no anomalies that could lead to an event of this scale.
“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 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.”
Mike Gubbins, head of the fish health inspectorate at Cefas, added: "We are continuing to investigate whether an aquatic animal disease has caused this mortality.
"Our fish health inspectorate have been analysing shellfish samples collected from the area for listed and other non-listed diseases, but none have been detected so far.
“We will continue to work with partner agencies to try and find answers for the local community.”
The North Eastern Inshore Fisheries and Conservation Authority (NEIFCA), the Marine Management Organisation (MMO), Hartlepool Borough Council and Redcar and Cleveland Borough Council are also involved in the investigation.