A fishing lake is to remain sealed off for at least a week after three dogs died and seven others fell sick.
Kent Police said the animals, which were all being walked by a minder, had been drinking from Brooklands Lake in Powder Mill Lane, Dartford.
The water has been “completely closed” to anglers and walkers while the Environment Agency runs tests.
The vets treating the dogs said algae poisoning was thought to be the cause. Police have ruled out foul play.
Parkvets surgery in Sidcup said blue-green algae produced a potential toxin at this time of year and throughout the summer.
“This means that water that was previously safe can suddenly become toxic,” said vet Emily Brady.
“There are also serious health risks to humans who come into contact with infected water.”
Initial symptoms, which include muscle tremors, vomiting, diarrhoea and seizures, can lead to death in as little as 15 minutes.
Lindsey Olney’s six-month-old boxer puppy, Stanley, was one of the dogs which died.
Ms Olney, from Bexleyheath, dropped her pet off with a dog minder on Tuesday morning and said she was “devastated and heartbroken”.
“They told me one of the other dogs started to have fits,” she said.
“So they put all of the dogs into the car then my puppy started fitting as well.
“They gave him CPR and drove him to the vets. They had their stomachs pumped.”
Dartford Borough Council leader Jeremy Kite said: “It is heartbreaking. Everyone knows how important dogs are to families.
“To lose one is appalling, to lose several in these circumstances is absolutely outrageous.”
A spokesman for Kent Police said no criminal offences had been committed and the matter was being handled by the council and the Environment Agency.
“The lake will remain closed for a least a week… and police are asking people to avoid the area,” he added.
The Environment Agency said water samples taken from the lake were being analysed in its laboratories and results were expected next week.
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It said it would “continue to support our partners, including Dartford Borough Council and Kent Police, as they work to understand the potential risk.”
Its officers have returned to the site to check on the lake and local wildlife.
Although it was not believed any fish had been harmed, fishing at the lake has been suspended.
People were also advised to stay away from the lake in 2013 when poisonous blue-green algae was found after a spell of hot and dry weather.