The BBC reports
More than 500 cannabis plants with a street value of £1m have been seized by police.
A drugs factory was discovered in Howardsgate, Welwyn Garden City, on 14 December, and Hertfordshire Police said officers were also treating it as a “modern slavery” inquiry.
The building’s mains supply had been tampered with, the force said.
A 42-year-old man from Hatfield was arrested at the scene on suspicion of producing a controlled drug.
He has been released under investigation.
Sgt Paul Brown said: “The cannabis factory is being treated as a case of modern slavery and we have been working with the Salvation Army in relation to safeguarding those involved.