Five men injured in the incident remain in hospital.
Police have issued an update on their on-going investigation into Wednesday night's explosion at a flat in Batley.
Detectives now believe it was caused by a 'large number of small flammable gas cylinders'.
They don't yet know how the cylinders came to ignite in the incident on Hick Lane, which took place at around 6.45pm.
Five men aged between 35 and 39 who suffered burn injuries remain in hospital.
Police say they've been working tirelessly with partner agencies to ensure the safety of the building and residents nearby.
A police cordon has remained in place in the nearby vicinity of the building whilst the investigation continues. Some roads around the area have since been reopened.
Detective Superintendent Pat Twiggs, from the force Homicide and Major Enquiry team who is leading the investigation, said:
“We were able to safely gain partial access to the building on Thursday afternoon and commence a detailed forensic search. We can confirm the explosion was not caused by a mains gas leak or any utility issue. A large number of small flammable gas cylinders in one of the flats is believed to be the source of the explosion.
"Enquiries and further forensic testing will be required to understand how the cylinders came to ignite and if any criminal offences have been committed. We anticipate being able to return the premises to the control of the owner today.
"I would like to thank all the residents and nearby businesses affected by the incident for their patience and cooperation whilst police and other emergency services have been conducting their enquires.”
Anyone who witnessed the incident or has any information is being urged to come forward and speak to the police via 101 quoting log number 1589 of 6 February.