Firefighters dealing with a severe blaze in a three-storey block of flats found 250 cannabis plants inside.
Crews were turned out to the fire in a converted terraced property in Leicester today and immediately mounted a search for possible casualties, according to Leicestershire Fire and Rescue Service.
However, firefighters carefully scouring the burning building soon discovered instead a large number of the illegal plants inside and called the police.
Nobody was hurt in the blaze i n Fosse Road North, and police have now launched a drugs investigation following the find.
Station manager Matt Cane, of Leicestershire Fire and Rescue Service, told the Leicester Mercury: "There was a severe fire on the first and second floors of the building, which is split into flats.
"We had eight teams, each with two firefighters wearing breathing apparatus (BA), searching the flats.
"The BA teams worked in arduous conditions, in high temperatures, searching the building to ensure there were no casualties inside.
"We were originally concerned that the occupant of one of the flats may still have been inside, but eventually all occupants were accounted for. The ambulance service stood by.
"A number of cannabis plants were found and the cause of the fire is believed to have been an electrical fault - there were several seats of fire."
Leicestershire Police said its officers were initially alerted by the fire brigade and had since counted "about 250 cannabis plants" in the house.
Nobody has been arrested but inquiries are continuing.
A force spokesman said: "We were called to reports of a fire on Fosse Road North at about 8.32am, today.
"When the fire crews attended they discovered a large number of plants of various sizes."
He said the plants had escaped fire damage, adding there was no health risk to anyone in the surrounding area.
"The fire service's investigation and a police investigation is continuing. At this time no arrests have been made," he said.