Two people were rescued from a fire in a ground-floor flat in Hove late last night (Thursday 8 September).
One man was taken to hospital for treatment after the entire building, in St Aubyns, Hove, was evacuated.
Sussex Police and East Sussex Fire and Rescue Service have started an investigation as the cause of the blaze was unclear.
Detective Sergeant Gary Tattersall said: “It is believed a group of four or five people were seen running away from the block of flats shortly before the blaze. Did you see them? Were you one of them?
“We would like to hear from anyone who was in the area at the time and who saw anything suspicious.”
The emergency services were called out just before 11.30pm. The fire service sent four fire engines from Preston Circus, in Brighton, Hove, Roedean and Eastbourne.
The aerial ladder platform was sent along with the command support unit from Lewes and the fire victim support unit.
The fire service said: “Two people were rescued from inside the ground-floor flat and crews wearing breathing apparatus used one hose reel and one main jet to tackle the fire.
“One man was treated at the scene and taken to hospital.”
Sussex Police urged witnesses to come forward, including the group seen running away from the flats.
The force said: “It is thought the blaze started in one of the ground-floor flats and the fire is currently being treated as unexplained.
“The block was evacuated but there were no serious injuries reported.”
Witnesses are asked to phone Sussex Police on 101 or email firstname.lastname@example.org quoting reference 1761 of 08/09.
Earlier, the fire service said, three appliances from Hove and Preston Circus were sent to a kitchen fire in Bramble Way, in Hollingbury, Brighton. No one was injured in the blaze which was reported just after 8pm.