The chemical cloud that caused a Sussex beach to be evacuated on Sunday might have come from a shipwreck, the coastguard agency has suggested.
Part of the East Sussex coast was engulfed by the cloud and about 150 people were treated for breathing problems, stinging eyes and vomiting.
The Maritime and Coastguard Agency (MCA) said emissions from the area's many shipwrecks might be the cause.
It is also investigating discharges from passing ships or lost cargo.
Birling Gap beach was evacuated after people began suffering unexplained symptoms from a "mist" that descended.
In the past, chemicals have drifted across from European industrial units, but Sussex Police said weather models suggested the source was unlikely to have been in northern France.
The MCA said in a statement: "As part of our investigations we are considering a number of possibilities, such as discharges from a vessel, previously unreported lost cargo, and emissions from known shipwrecks.
"We have identified approximately 180 vessels that passed through the English Channel off the coast of Eastbourne on Sunday.
"We are working with all relevant environmental and public health regulators to conclude these investigations. We have no further information at this stage."