Investigators looking into an explosion which ripped through an oil refinery are not ruling out human error as a cause.
More than 100 firefighters tackled the blaze at the 480-acre Conoco plant at South Killingholme, near Immingham, north Lincolnshire.
Although all 199 workers were later accounted for, fire chiefs warned things could have been far worse had it not been a Bank Holiday.
Investigators are preparing to sift through the debris for clues to the cause of the blast, focusing on a handful of scenarios.
A spokesman for the Health and Safety Executive said: "We will investigate and attempt to determine the cause of the incident and from there we will work back to do a combination of forensic work on the site and interviews with staff.
"If we can we will work back to see if anything could have or should have been done to minimise the risk of such an event occurring and whether other reasonable practicalities were taken.
"There will be investigators who have knowledge of the installation. They know what it does there and know how the processes work. They know the kinds of systems that go on there so there will be particular scenarios that people will concentrate on and work back on to see if there is evidence to support it.
"Scenarios will include human failing, hardware or software systems failing, a leak from a vessel or pipeline which led to a flammable mixture being formed in the air and that finding an ignition."
Witnesses said the blast shook buildings and the fire sent up a pall of thick black smoke which was visible for several miles.
Investigators were joined by scenes-of-crime officers and photographers on behalf of Humberside Police, who have ruled out foul play or sabotage.