The Environment Agency has said that the investigation continues almost four months after the incident was first reported at the start of October last year.
Hundreds of dead crabs had been spotted on the beach at Seaton Carew with the figure going into the thousands once similar remains were discovered along the Teesside and North Yorkshire coastline.
A number of possible causes have so far been ruled out, including chemical contamination and sewage and seismic activity.
The Environment Agency said that investigations are ongoing and Defra has taken on overall responsibility for the probe.
It said: “Investigations are ongoing. Defra has taken on overall responsibility for the investigation from the Environment Agency.
“The Environment Agency and our science advisors Cefas have undertaken extensive tests to try to determine the cause and are reviewing the evidence gathered since the start of this incident, as well as considering any additional work needed.
"The Environment Agency have not identified any chemical contamination in the area that is likely to have caused the crab and lobster mortalities.”
The president of the British Veterinary Association (BVA), Justine Shotton, has advised pet owners not to panic despite the spike in cases and to talk to their vets if there are any concerns.
Speaking to BBC Radio 4’s PM programme earlier this month, Dr Shotton highlighted that there was not enough evidence to say whether there are any possible links to beaches or other environmental factors.