The anonymous police source says to TV4 the investigation is now working on the assumption the fire was an accident, and may have been started inside the prayer room itself.
The police initially said they thought it was an attack, after a witness told them someone had thrown an object into the basement mosque. Police later withdrew that statement, but have not officially said they are no longer treating this as an arson case.
"There are a thousand ways to start a fire. At the moment, we do not know," Eskilstuna police spokesperson Lars Franzell told news agency AFP. A forensic report is due later this month.
Police still suspect arson in two other recent fires at Swedish mosques.