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."