Firefighters have left the scene of a huge fire which tore through one of the remaining sections of the West Pier in Brighton.
CCTV footage is being reviewed in a bid to find out how the fire started
The flames have left just the metal skeleton of the derelict pier standing.
The fire started in the old concert hall at about 0945 GMT on Friday and firefighters have said they believe it was started deliberately.
But Sussex Police say they are keeping an open mind about the cause until it is confirmed by fire investigators.
Earlier, Station Commander Phil Thompson, from Preston Circus Fire Station, said: "The fire is definitely of doubtful origin, there is no way it could have started on its own."
There were rumours a speedboat seen in the area early on Friday could be connected to the fire.
There must be suspicion, it is deliberate
Geoff Lockwood, West Pier Trust chief executive
But this was ruled out by the coastguard after a check of vessels entering and leaving Brighton Marina was carried out.
Police agreed and said they had traced the boat and eliminated it and those on board from their inquiries.
They said they were keen to trace any witnesses or anyone with information.
The force is reviewing CCTV footage in an effort to find out how the fire started.
Firefighters had fought the flames from a number of boats, unable to access the fire from land due to the gap in the middle section of the structure.
Firefighters found it impossible to reach the flames from land
They left the scene by 1500 GMT after an operation described as "effective".
An East Sussex Fire Brigade spokesman said the state of the tide and the loss of light had made it impossible to carry on the seaborne operation but that the brigade would continue to monitor the situation.
He said on Friday afternoon: "It is expected that the West Pier will continue to smoulder for some considerable time and it is not currently possible to place fire crews on the pier due to dangerous state of the structure."
Crowds of onlookers remained at the scene after the fire was brought under control, many taking photographs.
Fire will smoulder on
Security guards were deployed around the pier's entrance to keep the public away as a precaution.
Smoke from the fire began to engulf parts of Hove by mid-afternoon, blown there by a change in the direction of the wind.
Geoff Lockwood, chief executive of West Pier Trust, which owns the pier, said the fire would not hamper the redevelopment.
But he added: "We are puzzled by it. There's nothing combustible there, there is no electricity and there is no access from the land so how it started is a puzzle. There must be suspicion, it is deliberate."
The derelict West Pier is one of two piers in the city.
Brighton Pier was also hit by a fire in February which destroyed a fairground ride.
This fire comes only three months after part of the West Pier collapsed in the sea.
Part of the West Pier collapsed in December 2002
A walkway connecting the concert hall and pavilion fell into the sea in December after being battered by storms.
Bad weather caused further damage in January.
The pier - built in 1866 - has been in a state of disrepair for many years and has been closed to the public since 1975.
In February a £30m scheme to restore the derelict West Pier was approved by councillors at Brighton and Hove City Council.
Grade I listed pier
Although campaigners opposed to the scheme have vowed to fight for a public inquiry.
Many want to see the pier restored to its former glory but not in the design put forward by the West Pier Trust.
They say the planned development of cafes and bars at the promenade end of the Grade I listed pier would obstruct views of the sea.
On Friday Ken Bodfish, council leader said: "There was always going to be a substantial element of rebuilding rather than restoration.
"This will clearly mean far more rebuilding than envisaged."
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