The cause of the Pellon Lane mill fire in Halifax is expected by the end of this week.
Fire investigator Glenn Meachen said the state of the buildings was hampering efforts to determine the cause of the blaze.
At its peak, 200 firefighters from across West Yorkshire tackled the fire on Saturday afternoon.
Mr Meachen said: "There are a lot of people on site gathering information. Once we have all the information, from various sources, we'll be able to come to a conclusion, which could be by the end of the week."