Laundry has been found drying inside living quarters in a large cannabis factory in Leeds city centre.
The “industrial scale” growing operation – in a disused Chinese supermarket off Vicar Lane – was uncovered after an electrician turned up to check the supply on Tuesday.
The number of plants inside has been estimated at about 800 – with an approximate potential street value of £350,000.
Police remained at the four-storey former Wing Lee Hong business throughout Thursday as inquiries continued and forensic examinations were carried out.
Detectives have yet to arrest anyone, but have found evidence that at least one person was living in the building.
Det Insp Jaz Khan, from the Leeds police’s specialist drugs and guns team, Operation Quartz, said: “There are mattresses, washing lines and cooking equipment.
“A number of the plants are at the stage where they are being dried. It takes about three months for them to get to that stage, so this set-up has obviously been there for at least that amount of time.”
Police are following up a number of lines of enquiry resulting from an appeal for information, but want to hear from anyone who might have witnessed suspicious activity at the building.
Det Insp Khan said: “We suspect there will have been significant comings and goings around this building while this operation was set up,” he said.
“We would like to hear from anyone in the area, particularly nearby businesses, who may have seen anyone going to the building.
“We are committed to targeting criminals involved in the production and supply of illegal drugs that damage our communities and the public have a real part to play by passing on any information or suspicions they have so we can take action.”
Call Operation Quartz team via 101 or Crimestoppers, anonymously, on 0800 555 111.