The huge fire at a recycling plant on a Wellingborough industrial estate earlier this month was caused by sparks from an angle grinder.
A spokesman for Northamptonshire Fire and Rescue Service said this week that was the conclusion of fire investigation officers.
The blaze, at ESS Recycling in the Leyland Industrial Estate, started on the morning of Friday, April 4, burning for a number of days.
At its height, about 14 fire engines and 60 firefighters, from Northamptonshire, Leicestershire, Bedfordshire and Buckinghamshire, were called to deal with the fire.
No injuries were caused by the incident, but while the fire was still burning emergency services warned people living in the area to stay indoors and keep windows shut.