One of two men who were rescued from a fire at a cannabis factory has died in hospital.
Police believe the dead man, 43, and his companion had been locked into the industrial unit in Dudley, after being trafficked to the UK.
Officers appealed to the owner of a drone to come forward after it was seen in the area when the fire broke out.
The blaze at Premier Trading Estate, Leys Road, was discovered at about 19:30 BST on Thursday.
"The unit, used to grow cannabis on an industrial scale, was locked meaning these men had no way to escape the fire," West Midlands Police said in a statement.
The victim died in hospital on Friday afternoon and a post-mortem examination will take place to establish the cause of death.
The other man, aged in his 20s, has since been discharged from hospital.
The force said it believed both men had been "trafficked to the UK from abroad and were effectively being held against their will".
"The [drone] footage recorded may hold vital information in our investigation," said Det Insp Jim Mahon.
"We are urgently appealing for anyone with information about who is involved in exploiting these men, and involved in the set up, to contact us."
In order to make the site safe and recover evidence, power has been suspended to the trading estate, he added.