A teenager jumped from an upstairs window to escape from an arson attack on a house in Heckmondwike.
The 17-year-old boy suffered burns from the fire and an injury from the jump from the house on Stonedene Court in the early hours of Sunday.
Police, who were called at 4.13am, are treating the attack as attempted murder.
A woman and two girls, aged 16 and 11, also managed to escape the fire through windows of the house.
They were treated at the scene for smoke inhalation and taken to hospital. The 17-year-old is currently in a serious but stable condition in hospital being treated for his injuries.
Detectives are conducting active enquiries today and officers are also conducting reassurance patrols in the area.
Detective Superintendent Mark Swift, of the Homicide and Major Enquiry Team - who is leading the investigation, said someone could have died.
“This is clearly a very serious incident in which a male has suffered burn injuries and three females have also suffered smoke inhalation.
“It took place at a time of night when a family were at home sleeping and could quite easily have had fatal consequences for those inside.
“We are treating this incident as an attempted murder and I am appealing to members of the public for information.
“It is quite possible the suspects will have suffered some injuries as a result of lighting the fire and I would like to speak to anyone who has information about males having turned up at home this morning with burns.
"I would also like anyone who has information about males buying petrol in cans from garages to contact us.
“Also, anyone who saw suspicious behaviour in the area of the affected property, or who has CCTV of such behaviour, should get in touch.”
Anyone who has information is asked to contact the Homicide and Major Enquiry Team, (HMET) on 101 referencing Operation PAIRGLEN and crime number 13190409679.
Information can also be given online at www.westyorkshire.police.uk/101livechat or anonymously to the independent Crimestoppers charity on 0800 555 111.