A substance used to poison five dogs and which killed two of them has been identified by police.
Officers investigating the poisonings in Fryent Country Park say the substance used to poison pets was found to be Methomyl, an insecticide which has been banned due to its high toxicity.
Police and animal welfare charity the RSPCA are continuing their appeal for more information to find those leaving the substance in the parks. No arrests have been made.
Last month the Harrow Times reported two dogs had been killed after visiting the park.
RSPCA chief inspector Simon Osborne said: "This kind of poison is extremely toxic for dogs and other animals if consumed and it is very concerning to hear that this is what caused the recent deaths.
"We urge anyone who knows of any person who has a stock of this particular insecticide at home or in even their shed or garage to talk the police or ourselves in complete confidence, so we can investigate further."
Sergeant Peter Rowntree said: “I am grateful to everyone who has assisted to raise awareness regarding these incidents and I would encourage all dog walkers and the users of any of our parks, to be vigilant and phone through any suspicious activity.
“With better weather and longer hours of daylight the parks will be busier and I am hopeful that any criminal activity is more likely to be seen and phoned through to us."