Police were informed by East Sussex Fire and Rescue that a small fire at St Leonards-on-sea could have been started deliberately.
Emergency services attended the block of flats in Cave Road at around 11pm.
A small fire was located on the stairwell and quickly extinguished.
Detective Sergeant Tony Webb, from East Sussex CID said: "Although this was a small fire and fortunately no one was injured but it could have ended with more serious consequences. Officers will be continuing with local enquiries today."
More top news
Thursday morning's weather for the east of the region
Thursday morning's weather for the west of the region
Wednesday evening's Pollen Forecast