The culprit behind the unexplained mass death of crustaceans off the coast of northeast England over the last two years has left a panel of experts baffled as they failed to identify a single clear cause for the deaths of thousands of crabs and lobsters.

Dead and dying crabs and lobsters began washing up en masse at beaches in the Teesside region of northeast England in October 2021, drawing national attention as local fishing communities suffered and some feared a mass poisoning linked to the area's industrial past.

Fishing groups said decades-old toxic chemicals could have been released by dredging activity and were skeptical of an initial investigation by the environment department that suggested the deaths were likely caused by a natural but devastating algal bloom.

In a long-awaited report, a panel of independent experts convened by the British government said both those theories were unlikely, and that it had been "unable to identify a clear and convincing single cause" for the deaths.

"We've ruled things out, but we've not been able to confidently rule things in," panel member Tammy Horton, a researcher at the National Oceanography Centre, told an online press briefing. "We've come up with probably more questions than answers."

The report said the deaths occurred over a sustained period. It occurred along dozens of miles of coastline, involved unusual twitching of many crabs as they died and other marine life remained largely unaffected, it also said.

It was "about as likely as not" that a pathogen new to U.K. waters caused the deaths, it said, though no new pathogen was detected.

Several of the factors it considered might in combination also have been a cause, it added.

British Environment Minister Therese Coffey said on Friday she would consider if further analysis by government scientists might establish a conclusive cause.

"The focus now needs to be on understanding more about the novel pathogen identified as the most likely cause, and what support we can offer our local fishing industry as it recovers," Simon Clarke, a former Cabinet minister and the local lawmaker in the region, said.