A flat used by Manchester bomber Salman Abedi was found by its landlord with a stench of chemicals and disabled smoke alarms, a friend said.
The 22-year-old mass murderer was thought to have stayed in the flat in Blackley, north of Manchester city centre, until around six weeks before his attack.
Landlord Aimen Elwafi found the property with children's stickers scribbled out on the walls, a metal rod in the bath, material cut up and the electricity turned off, friend Mohammed El-Hudarey told the BBC.
A window in the flat appeared not to have been opened for two months, he added.
It fuelled speculation Abedi used the tower block property, which the landlord allegedly sublet despite it breaching his tenancy agreement, to build the device which slaughtered crowds on Monday.
But the discovery did not provoke alarm at the time, with the landlord suspecting Abedi might have been practising black magic.
"We didn't even think 1% he was a terrorist or a bomb-maker. We thought he must have been a drug dealer or doing witchcraft," Mr El-Hudarey told the broadcaster.
Abedi moved into the address around three-and-a-half months ago, he added, later claiming in a late-night call he was "flying abroad" and would be moving out.
Mr Elwafi was said to have only realised his tenant's true intentions when his identity was published in the wake of the atrocity.
He went to police with his information, having reportedly been misled by the British-born bomber, who claimed he was a student and delivery driver.
"He was shocked and in a bad situation. Very upset. There were tears coming from his eyes," the friend said.