Police say witnesses reported people lighting a bonfire on the moorland above Stalybridge, Greater Manchester, on the early evening of Sunday, 24 June - about 50 minutes before the first emergency call to raise the alarm.
The fire on land at Buckton Vale, east of Manchester, later led two days later to the evacuation of dozens of homes in the village of Carrbrook as flames threatened to engulf properties and continued to spread.
Fire crews from across the country, with the assistance of the army, have successfully battled to keep the area safe.
With the fire still burning today, although under control, Greater Manchester Police said the bonfire sighting was being pursued as a possible line of inquiry.
Chief Superintendent Neil Evans said: “The moorland fire is now being investigated as arson following information from local witnesses and initial inquiries.
“The scale of this fire is exceptional and we are treating it with the utmost seriousness.
“That said, we are under no illusion that solid evidence as to where the fire started and what was the cause will not be easy to establish.”
Mr Evans said: “The areas we have been able to look include dusty ash, burnt peat and grass and because of this our work with the fire service will be painstaking.
“We recognise the incredible support that the emergency services have received and we now hope that the local community can help us further by getting in touch with any information that could help our investigation.”
The bonfire sighting was about 7:30pm on 24 June, with the fire service called out at 8:19pm.
Greater Manchester Fire and Rescue Service (GMFRS) interim chief officer Dawn Docx said: “Specialist fire investigation officers at Greater Manchester Fire and Rescue Service are working closely with Greater Manchester Police to investigate the suspected arson in the moorlands in Tameside.
“Investigations are currently ongoing with a multi-agency approach to find the source of the ignition. There is an area of interest that specialist officers are inspecting.
“Firefighters are working in extremely difficult and challenging circumstances in Tameside and I can’t praise crews enough. They’ve done a fantastic job and are continuing to work hard to put out the multiple fires across the moorlands.
“I would like to urge the public to be really careful with barbecues, discarded cigarettes and glass bottles in the countryside. Fires in the moorlands can spread rapidly so it’s important that we take extra care.”
Soldiers drafted in to help Greater Manchester firefighters tackle the Saddleworth Moor blaze will continue their temporary roles until the end of the week.
Around 100 troops from the Royal Regiment of Scotland have been involved in the operation in Tameside, including digging trenches in an effort to stop the moorland fires spreading.
There were 45 firefighters on the scene today who were using specialist equipment and vehicles to extinguish the remaining pockets of fire, said GMFRS.
Firefighters are being supporting by two helicopters from United Utilities that are continuing to dampen down targeted areas.