A huge cannabis farm containing more than 2,000 plants was discovered after a raid on a property in Bradford.
West Yorkshire Police set up a cordon on Morley Street, Bradford, on 21 September after reports that "potentially hazardous materials" might be on site.
The force said no such materials were found but officers seized more than 2,000 plants worth a potential value of £300,000.
Enquiries are ongoing to find out who used the building.
Insp Richard Baildon of Bradford City Neighbourhood Policing Team asked anyone who had information to get in touch.
He said: "Drugs have no place in our community, and I would like that thank those people who came forward to report suspicious activity about drug crime."