Tewksbury firefighters battled a stubborn fire in a large commercial building. Companies arrived to find heavy fire showing from 1 bay in a 200x50 commercial building. Command struck a 2nd alarm bringing in mutual aid to the scene and to cover. Companies stretched multiple handlines and gained access to the other bays to start knocking down the volume of fire inside. Command struck a 3rd alarm bringing in additional mutual aid to the scene and to cover. Multiple master streams and 2 1/2 handlines were used to knock down the heavy fire. Companies remained on scene overhauling for several hours.