From the moment the first flames licked the air, the question has been: Why would someone set fire to the West Pier?

Rumours are flying as thick and as fast as the ash-grey smoke that billowed from the stricken structure.

Few are considering an innocent explanation.

After all, damp empty buildings don't just burst into flames.

People are claiming to have seen a firework or flare being launched at the structure.

This theory seems to be supported by witness accounts of the pier smouldering then suddenly bursting into flames.

Ben Smith, manager of Mr Munch, a cafe about 50 yards from the West Pier, said: "The fire certainly spread very, very quickly.

"I don't see how a building that was damp and neglected and wet could suddenly catch fire.

"There was smoke coming out of the pier and then all of a sudden it just exploded.

"I was parking by the West Pier at about 9am and saw a black 21ft speedboat come along at high speed. Perhaps it was a just a coincidence."

Last night police had ruled out any connection with the boat, after tracing its owners from CCTV footage. The boat's occupants were sightseers on their way to Brighton Marina.

Regardless of how it was done, the main worry for those who hope to restore the West Pier is why someone would do it at all.

Surely adding to the misery of this stately old lady, already buckled and forlorn, was just an act of sheer cruelty?

Rachel Clark, general manager of the West Pier Trust, said: "I don't understand it at all.

"The one thing I am very aware of is that whoever set fire to it would have put themselves in a lot of danger.

"That confuses me because they must have been very determined.

"At least when the pier collapsed a few months ago it was a natural event but something that is started deliberately is a much more frightening event."

In a normal criminal case, suspicions turns to those with motive but even the most ardent opponents of the redevelopment of the West Pier cannot believe it was targeted by an arsonist.

Sue Paskins, of Save Our Seafront, said: "It seems amazing that the Palace Pier had a fire and then the West Pier did too. Have we got someone who just wants to burn piers?

"I hope the police catch them and they don't just get a slap on the wrist either."

The police are not confident the precise cause or motive will ever come to light.

Much of the debris which could hold vital clues either perished or fell into the sea.

Investigators were continuing last night and detectives moved rapidly to eliminate the mystery speedboat.

Fire chiefs say the cause was doubtful - which could mean arson or an accident - and police said they were keeping an open mind.

Detective Chief Inspector Martin Cheesman, head of Brighton and Hove CID, said: "We have had no allegations the fire was started deliberately but we are investigating and we would appeal to anyone with any information to come forward."