A mother and her two sons, whose bodies were found in a bedroom at their home, had been stabbed, police said today.
Clair Smith, 36, and her sons Ben, nine, and Aaron, one, are thought to have been attacked by the boys' father Richard Smith, 37, who then killed himself.
Police said today all four were found dead together on a double bed in the fire-damaged main bedroom of their home near Leeds, on Sunday.
Officers confirmed Richard Smith died from smoke inhalation.
They said Mrs Smith died from a single stab wound to her chest as did baby Aaron.
Ben suffered a stab wound to the chest area but died as a result of suffocation.
Today Detective Superintendent Paul Taylor said Ben's unopened advent calendar was one clue to when the killings happened.
Mr Taylor said the family were last seen at 11pm on Friday night when they left a function at a cricket club by taxi.
The detectives said: "Ben's advent calendar is not opened on the Saturday so we believe something's happened overnight Friday or the early hours of Saturday but, of course, if anybody else has any information that would contradict that we would very much want to hear from them."
Mr Taylor said officers were talking to people who were at the cricket club party but confirmed there had been no reports of any unusual incidents at the function.
The family was found dead after concerned neighbours called at the house on Sunday afternoon followed soon after by police officers.
Police have not commented on reports Smith was seen cleaning his car at the weekend.
Mr Taylor said he could not give any further details about how the family died.
He repeated that due to the "cause of deaths and other physical evidence found with the scene" he is not looking for anyone else in connection with the deaths.
The detective said a number of items had been found in the house which could be used as weapons and they are being analysed.
He said he did not want to speculate about what prompted the killings.
"The coroner must rule on what happened that night or over that weekend. You can draw conclusions on what I've said if you wish but I'll be putting a file to the coroner."
Mr Taylor said: "I really don't want to go into anything about motive or anything like that.
"That'll be the thrust of the ongoing investigation that's going to take place over the next few months.
"What the state of mind of Richard, Clair or anybody else was will be a matter we'll be trying to investigate.
"It's not for me to guess now."
Tributes have continued to be left at the police cordon on Sheridan Way, which is about 50 yards from the house.
Mrs Smith's parents, Alan and Norma Robinson, released as statement which said: "Clair was an exceptional person, loving and caring to everyone.
"She was a loving mum, her children were her life, and everything centred around her family and home.
"She was our daughter and our best pal. We cannot imagine what caused this tragedy."
Yesterday, relatives described the Smiths as a "perfect family".
Friends said Smith worked for a shopfitting firm as a designer and was a coach for a local football club.
They said the family appeared to have no problems and could not understand what had happened.