Police in north London are asking dog owners to be careful following a spate of dog poisoning in the area.
Police in Brent and the RSPCA have put out a warning after three dogs died and four more became ill through poisoning since the start of the year.
In the first incident, on 21 January, a dog died after eating blue tablets near a lay-by on Fryent Way, in the Fryent Country Park, Kingsbury.
On 13 March, one dog died and two more became ill after eating discarded meat which had been contaminated with poison near the Fryent Country Park roundabout.
On 14 March, two more dogs ate poisoned meat that had been placed in undergrowth in Salmon Street. One dog died and the other survived after being treated by a vet.
Officers from Brent borough are investigating what they describe as a "nasty, cruel act".
Simon Osborne, chief inspector for the RSPCA, said: "We are very concerned to hear these reports. Poisoning is a cruel and inhumane way for an animal to die and one which would have caused these dogs a great deal of suffering.
"The RSPCA has offered the police any support they need in dealing with these incidents and urges anyone with any information to come forward."
A Natural England spokesperson said: "This is a very serious incident and clearly very distressing for dog-owners in the area. We will be assisting the Met, under the Wildlife Incident Investigation Scheme, to determine whether pesticides are involved."
Sgt Peter Rowntree from Brent Police said: "This is such a nasty, cruel act which has killed three beloved dogs and endangered all domestic pets and wild animals, in such close proximity to a country park.
"Family pets, wildlife and the amenities of a country park are factors that enhance the quality of people's lives. The owners and families of the dogs affected have suffered acute distress.
"This is not simple, ignorant mischief, but criminal activity and the persons found to be responsible will be prosecuted."
Police added that poisoned dogs would show symptoms including vomiting, diarrhoea, increased urination, excessive salivation and difficulty in breathing. Less notable symptoms may also include contraction or dilation of pupils and tearing of the eyes.