Fire crews and police evacuated residents of flats near to a blaze in Preston in the early hours of today.
Officers say they are treating the fire in the doorway of a ground floor apartment in Avenham Road as suspicious.
Two crews were called to the incident at shortly after 2am following a 999 call from a neighbour. Five people from adjacent properties had to be evacuated because of fears of a gas leak.
Firefighters wearing breathing apparatus put out the blaze which caused damage to the doorway.
A fire brigade spokesperson said later: "We requested the attendance of the police because we believe the fire was suspicious. We also requested the attendance of the gas company.
"Five people were evacuated from neighbouring flats as a precaution. There was a smell of gas and that caused concern because it registered on the detectors on our appliances.
"Once the fire was out we left the incident in the hands of the police and the gas board.
"We have returned to the scene this morning to carry out further investigations."