Nelson BMX Club's two-storeyed viewing platform was destroyed in a suspicious fire at 4am today.
It was one of two suspected arsons, with the Red Devils' Nelson headquarters' fence also set alight early this morning.
Nelson Fire Service referred the BMX club Beach Rd fire to the police to investigate because of its timing and location, said station officer Brian O'Donnell.
Sergeant Mike Wardle, of Nelson, said by the time the fire was spotted, it was well advanced and the structure collapsed as emergency services were called.
Tahunanui resident Jonathan McKeown, who was at home watching cricket, said he heard a high-pitched noise, looked out his window and saw the blaze which had engulfed the two-storeyed platform and storage facility. He called emergency services, which had already been alerted.
Mr O'Donnell said there was no hope of saving it.
Nelson BMX Club members were shocked by the fire at the track, said president Chris Jones.
"We're a very small club and the news that we've lost all our maintenance equipment, trophies and historical records is devastating."
About 30 youngsters attend club sessions, but about 100 individuals use the track each week.
The damage comes in the middle of the racing season.
"Some of our riders are achieving very well nationally, but they will probably have to miss their final training sessions before the South Island Championships in Christchurch next week.
"We'll struggle to fund replacements before the National Championships at Easter, and this will impact on the kids, too," Mr Jones said.
The firefighters were called at 3.15am to the Natalie St headquarters of the Red Devils motorcycle club.
Mr O'Donnell said two people were in the house, but they were unaffected by a fire which scorched a fence and garage.
New Zealand Post staff spotted the fire.
Mr Wardle said the police would be making inquiries.
The Red Devils have been involved in a high-profile case in which prosecutions have been stayed against people caught in a police undercover operation, and the Crown is appealing the High Court judgment.
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