A chemical mist which led to 150 people being hospitalised after it drifted along England's south coast is being investigated by the government.
The gas caused breathing problems, stinging eyes and vomiting when it descended on a beach in East Sussex, which had to be evacuated, in August.
A sample of the hazardous cloud - first reported in Birling Gap - was not taken and its cause never found.
The government's operational response will be reviewed.
"This was an unprecedented event and despite a comprehensive investigation it has not been possible to identify a specific source of the gas cloud," a Department for Environment, Food and Rural Affairs (Defra) spokesman said.
"Given the meteorological evidence, the most likely source was a passing ship, lost cargo or a wreck in the English Channel."
'Get on with it'
However, it added a sample was not taken for analysis as "appropriate equipment was not available at the site and the gas cloud dispersed quickly".
It will now review "all aspects" of the incident.
Eastbourne MP Stephen Lloyd said discussions with "various bodies" suggested the cause of the mist was down to "a passing ship doing an illegal clean-out with bleach".
Andrew Durling, from Eastbourne Friends of the Earth, said: "It's well known in the shipping world that ships illegally bleach their tanks at sea. It could well happen again.
"The authorities should have had a permanent network of air monitoring stations along the coast, ready to capture immediately the data needed to determine the exact cause."
"It could have been so much worse, people could have died. If you are serious about protecting people's health, find the money and get on with it."