No working smoke alarms were found in the house where a fire left two young boys dead, the fire service has said.
Logan Taylor, three, and two-year-old Jake Casey were rushed to hospital on Saturday afternoon after the blaze in Huddersfield erupted but could not be saved.
Their mother and an older brother managed to flee the house in Alder Street before firefighters arrived.
Deputy chief fire officer Dave Walton said: "We can confirm that at the time of the fire, there were no working smoke alarms in the property.
"Further investigations are ongoing to determine the exact circumstances but we urge people to always have a working smoke alarm and to test it regularly."
He added: "Whilst early indications are that the cause of the fire is not suspicious, investigations are ongoing and it may be some days before we have a conclusive cause of the fire."
The fire was believed to have started in the front bedroom and a rescuer, thought to be a neighbour, tried to enter by climbing on a veranda at the back of the house.
The boys' father also dashed to the house, which the family had moved into around six months ago, to try to help, neighbours said.
The family's next door neighbour, who did not want to be named, said the mother was shouting, "Help me! Help me!", crying and screaming for assistance.
"We saw the smoke everywhere, a man tried to climb up to help but there was too much smoke, you could not see anything," she said.
"I asked the man, he said two children were still inside. Then the police and fire brigade came and they went inside.
"It's really shocking, everyone is in shock. My children were upset."
Family members have returned to the scene of the tragedy to leave flowers.
Fire crews will be in the area over the coming days offering fire safety advice and fitting smoke alarms where required.
West Yorkshire Police said the fire appeared to be a "tragic accident" but the investigation was continuing.