The reason why several people started coughing and struggling to breathe on an Essex beach remains a mystery.
Hundreds of families had been enjoying the warm late August bank holiday weekend at the seafront off Fourth Avenue in Frinton on Sunday afternoon when a large number of people started to feel unwell.
People said they had been left struggling to breathe and police advised people not to go into the sea.
There was speculation that a fuel spill had occurred, but the Maritime and Coastguard Agency said there was no immediate evidence after it sent a counter-pollution aircraft to the scene.
Essex Police is investigating but the cause remains unknown.
Miriam Lansdell, who was visiting her parents in Essex, said: “My daughter started coughing. She said ‘I don’t feel good, it hurts to breathe in’.
“My other daughter was gasping and couldn’t form words because she couldn’t breathe well enough.”
The mental health worker, 45, said she also had problems breathing as she lay on the sand after swimming.
When they moved further away from the beach they began to feel better but she took her 10-year-old girls to a clinic to be checked.
Her father had been told by someone in a speedboat that there may have been a fuel spill, Ms Lansdell said.
She said: “My dad said he had been asked to get out of the water by a man on a boat. He asked why and the man said there had been a fuel spill. He said if anyone is having breathing difficulties they should probably call an ambulance.
“It’s not what you expect when you go for a day out to the beach.”
One person tweeted that there were “lots of people coughing heavily”.
A mother said her son began coughing after swimming and had to be given his inhaler.
Another said: “We have just left Frinton and have seen lots of fire engines on the way out. Has there been an incident? We were on the beach and all developed a cough and were struggling to breathe.”
Training manager, Mark Wray, who was at the beach with his wife, said he had noticed children coughing as they came out of the sea, but he assumed they had swallowed some water.
East of England Ambulance Service said people who were in the water should wash themselves down, change their clothes and drink fresh water.