They say there is nothing at present to suggest it was malicious.
A shop was badly damaged in the fire, and surrounding properties had to be evacuated.
The fire centred around the Pound Shop at Railway Street in Antrim, which was sealed off by police.
Around 40 members of the Fire and Rescue Service were called in to tackle the outbreak around 9pm last night.
The shop was destroyed, but firefighters prevented the fire from reaching neighbouring buildings.
An amusement arcade and some other nearby premises had to be evacuated.
