Cleveland Police officers were called to the property in Grangetown, Middlesbrough, during the early hours of the morning.
The force said it was investigating what had happened but was "keeping an open mind and cannot rule out any links to wider organised crime at this time".
A woman is in hospital in a critical condition, alongside a further three people who suffered non-life threatening injuries.
The fire happened about two miles from the A66 crash that killed five young men and two police officers on Saturday morning.
Redcar and Cleveland Local Policing Commander Supt Emily Harrison said the force was working to "establish the circumstances of exactly what has happened".
The families of the people involved have been informed and they are being supported by specialist officers.
This breaking news story is being updated and more details will be published shortly. Please refresh the page for the fullest version.
You can receive Breaking News on a smartphone or tablet via the BBC News App. You can also follow @BBCBreaking on X, external to get the latest alerts.
