Much of the evidence was lost to sea the report said
A blaze which destroyed the pavilion of Weston-super-Mare's Grand Pier was probably started by an electrical fault in the north east tower.
However an official report by Avon and Somerset Fire and Rescue Service concluded the official cause of the blaze would be recorded as unknown.
It ruled out arson but said the job was made much harder because much of the evidence had been lost to the sea.
In a statement the owners of the pier thanked the brigade for its efforts.
Co-owner Kerry Michael said: "Michelle and I would like to thank the fire brigade and forensic scientists for all their efforts on the day and for their diligent interrogation into the possible causes of the fire.
"Whilst it has not been possible to confirm the cause with any certainty, it is somewhat reassuring to know that certain things have been ruled out, for example speculation suggesting a kitchen fire have been laid to rest."
The blaze is the second time the pier has burned down; the first was in 1930 when the rebuilding process took three years.
Then it took three years for the pavilion to be rebuilt.
At a news conference on Wednesday Avon Fire and Rescue said after trawling through CCTV footage, taking eye-witness accounts and examining the scene, its investigation was inconclusive.
Deputy chief fire officer Jerry O'Brien said: "Taking into consideration evidence provided by eye witnesses CCTV images, firefighters and a detailed scene examination, the cause of this fire will therefore officially be recorded as unknown."
Mr O'Brien said arson was unlikely as there was "no evidence of forced entry or use of accelerants".
He said the most likely cause was "an electrical fault of unknown origin above ground floor level in the north east corner of the Pavilion - where the fire started but the investigation did not provide enough evidence for this to be a conclusive finding."
"A number of potential causes were identified, meticulously analysed and systematically discounted by forensic experts during the investigation.
"It is therefore considered unlikely the fire was caused by carelessly discarded smoking materials, electrical cooking or heating appliances, self-heating of cooking substances, a gas supply or source of naked flame."
Mr O'Brien said the fire service may have had a better chance of defeating the blaze if an alarm triggered at 0135 BST was brought to its attention.
"The alarm - which went off five hours before the service was alerted - was connected to an alarm monitoring company which failed to contact Avon Fire and Rescue Service."
The quarter-of-a-mile-long pier, which attracts thousands of visitors a year, reopened in April after a massive revamp.
It first opened in 1904 and was later bought by the Brenner family who sold it in a multi-million pound deal in February to brother and sister Michelle and Kerry Michael.
The Michaels had already spent £1m installing a go-kart track, a climbing wall and bar before the blaze.
"Now that the fire report has been published it enables us to close that chapter and move on with the important job of rebuilding the pavilion," said Kerry Michael.
"We are looking forward to provide a state-of-the-art 21st Century facility for our town.
"Our vision is that along with the redevelopment of the Tropicana, this can form a catalyst for the regeneration of the town."