Hundreds of cannabis plants were found at a flat in Wakefield after firefighters were called to a suspected arson attack at the property.
The fire service alerted police after finding the plants and hydroponic equipment in the run-down building in Balne Lane on Wednesday evening.
After the fire was extinguished, police seized "several hundred" plants.
Det Insp Mabs Hussain, of the Wakefield District Drugs Team, said: "This is a significant amount of cannabis."
He urged anyone who had seen "unusual comings and goings" at the property to contact West Yorkshire Police.
Landlords warned
"The property was rented and we would advise landlords to be on their guard and report anything unusual to police," he said.
"Tenants may be running up unusually high electricity bills and they may be reluctant to put bills in their name.
"If they tamper with the electricity they can also cause significant damage to the property which ultimately the landlords will be responsible for."
A West Yorkshire Fire & Rescue Service spokesman said the fire was believed to have been started deliberately and the cause was being investigated.