Three dogs have died and several others are unwell after drinking from a fishing lake in Kent, police have said.
Brooklands Lake in Powder Mill Lane, Dartford has been cordoned off and the Environment Agency notified.
Vets are working to save the life of a fourth dog which became unwell after entering the lake. It is understood the dogs were all being walked together.
It is thought a local vet is treating seven dogs in all and the animals have "unidentified poisoning".
Police said they were warning people to stay away from the lakes as a precaution.
It has not yet been confirmed whether the dogs became ill due to water from the lake.
Dog owner Chris Barnett said her Jack Russell, Millie, was taken ill after walking at the lake last week.
"She picked something up and she was so ill all night but the next day she was OK," she said.
"I have heard somebody is putting stuff down for dogs to eat.
"I am certainly not going there for the next few weeks until things are cleared up."
The Environment Agency said water samples were being taken but it was not believed any fish had been harmed.
A spokesman said officials were warning anglers to stay away until more was known about the incident.
Brooklands Lakes is a fishing venue run by Dartford Borough Council.
The council said parts of the lakes and associated footpaths were closed to pedestrians and dog walkers while the water was tested.
It said it was likely they would remain shut until the results were known.
People were also advised to stay away from the lake in 2013 after blue-green algae was found after a spell of hot and dry weather.
The council said at the time the algae was potentially harmful to humans and toxic to animals.