A hazardous material that left two people on Worthing seafront requiring hospital treatment might have been caused by a passing cargo ship, according to the coastguard.
Several people complained of sore eyes and vomiting at about midday on Sunday.
Two people were taken to hospital and later released after treatment.
A section of the beach was also cordoned off and evacuated.
People were told to avoid the area and lock windows and doors with the all-clear being given at about 4pm.
East Worthing and Shoreham MP Tim Loughton wrote on Twitter the investigation was in response to reports of "bathers affected by some sort of chemical type irritants".
A spokeswoman for the Maritime and Coastguard Agency (MCA) said it was investigating with the focus on the possibility of a gas-carrying or cargo ship.
She said: "The Maritime & Coastguard Agency's Counter Pollution and Salvage Branch has been informed and is monitoring the situation along with the Local Authority and Environment Agency.
"At this time we do not know the source and the MCA is investigating if the source could have been from a vessel.
"The MCA is also looking at weather patterns and vessel movements to establish if there were any vessels in the area that were capable of carrying a gas or could have cargo tanks (carrying solid or liquid cargo) which could have been venting at the time."
A Sussex Police spokesman said: "A small number of people have reported symptoms including sore eyes and vomiting, two of whom were taken to hospital. They have both since been discharged, and advised to go home, wash their clothes and have a shower.
"Anyone else affected is advised to do the same - and use copious amounts of water to wash your eyes."
The force added: "We'd like to thank everyone for their patience and understanding during the incident. We are following up a number of lines of enquiry and the investigation into what happened is ongoing."
An investigation was launched in August 2017 when a chemical haze in Eastbourne left 150 people needing hospital treatment for sore eyes and breathing problems.
The most likely cause of that incident was believed to have been fumes from a ship illegally flushing its tanks with chemicals.
