Flares were seen being fired underneath Brighton's West Pier shortly before it was engulfed by fire, it has been reported.
The cause of the fire on the West Pier may never be known
Police are investigating after a man who had been waterskiing near to the pier contacted them to say he had seen someone in a dinghy firing upwards towards the derelict structure.
A massive fire tore through part of the Victorian pier on 28 March leaving just a metal skeleton at the pavilion end.
Firefighters had to watch from the shore as the fire burned because it was too difficult and dangerous to reach.
Detective Chief Inspector Steve Fowler from Brighton and Hove CID said: "We are receiving a number of different reports all of which are currently being followed up as part of the arson investigation."
More than 1,000 people stood on the beach to watch the spectacle of the pier burning.
But an officer from East Sussex Fire Brigade said the fire had made the 137-year-old pier so unsafe it is too risky to send fire investigation officers on to it and so the cause may never be known.
A £30m scheme to restore the pier, which has been closed to the public since 1975, is still going ahead as planned after it was agreed by Brighton and Hove City Council.