Two women and two children were rescued by firefighters following an arson attack on a flat in Suffolk.
Emergency services were called to Whitland Close, Ipswich, in the early hours.
A joint fire service and police investigation is taking place and it is thought an accelerant was poured through the letterbox and set alight.
Det Insp Stuart Sowerby said: "This is a nasty incident that could have had serious, even fatal, consequences."
Anyone who was in the Whitland Close area in the early hours or anyone who may have information is asked to contact police.