Continuing hot spots could hamper investigators trying to find out what caused a fire that raged through four streets of bars, pizza shops and T-shirt stands - around 50 businesses in all - on a Jersey shore boardwalk still trying to recover from Superstorm Sandy.

The initial problem is spots where flames could keep flaring up among the rubble from the blaze that shot fireballs 50ft into the sky on Thursday night before crews could contain the fire.

In a news briefing, New Jersey governor Chris Christie said it would be "irresponsible for any of us to speculate" what sparked the fire that is believed to have started in or near a frozen custard stand at Seaside Park.

He said firefighters working on spraying down any flare-ups were also doing their best to preserve evidence for the fire investigation team trying to piece together what happened.

In the meantime, plans to rebuild again already are in the works.

Bob Martucci, the borough administrator in Seaside Park, said it would cost 600,000 dollars to rebuild the borough-owned boardwalk that was burned in his town. But that would cover just the stretch of boardwalk itself, not the businesses that were ruined.

The boardwalk itself, battered by Sandy last October, was rebuilt five months ago. One of the big attractions in the stretch - the Funtown amusement pier - had not reopened after Sandy. It burned in the fire, with its landmark Funtown Pier sign collapsing in a cloud of flames and sparks, falling on the gray charred remains of a once colourful attraction.

Among the businesses destroyed were a fudge shop, fry stands and a bar and grill where Nicole "Snooki" Polizzi was punched in the face during filming of the first season of MTV's Jersey Shore.