New Jersey Gov. Chris Christie said the cause of the blaze was still unknown, The New York Times reported.
"We honestly have no idea," Christie said at a news conference. "It is not even 24 hours since the fire started. We spent all of our time and resources on fighting the fire."
The fire began Thursday afternoon in an ice cream shop in Seaside Park, a tiny slice of barrier island in Ocean County. It got under the boardwalk and spread north, whipped by high winds, into the better known town of Seaside Heights before it was brought under control Friday.
About 80 percent of the Seaside Park boardwalk was destroyed. Seaside Heights lost at least six businesses and part of its boardwalk, including sections firefighters ripped up to create trenches in an effort to halt the blaze.
The area was devastated in October 2012 by Superstorm Sandy.
Christie said three firefighters, who apparently fell from a National Guard truck, were seriously hurt, The Philadelphia Inquirer reported.
As firefighters doused hot spots, Daniel Shauger looked toward his business, the Funtown Park Arcade -- unable to get close enough to determine whether anything survived.
"Everyone is struggling down here, and now everything is completely destroyed," he told the Times. "I am sure everything inside has to be replaced, and somehow it will be. We have a few tough years ahead of us but we will come back."
Christie said he told his staff he felt "like I want to throw up," considering how much work people in the area did to rebuild after Sandy.
More than 400 firefighters from more than 30 towns in seven New Jersey counties battled the blaze, using water pumped in from a bay five blocks west.
The fire's spread was accelerated by winds gusting to 40 mph, Seaside Heights Police Chief Thomas Boyd said. Embers blew for at least eight blocks, igniting a fire at a nearby condominium complex that was quickly doused.
The 84-year-old Funtown Pier amusement park, which was expected to reopen next year after being ravaged by Sandy, collapsed in flames.
The park's historic carousel, which was painstakingly restored after the storm and reopened just months ago, appeared ruined, the Times reported.
"The only building that's not consumed by fire is part of the arcade and the Saw Mill Cafe," Ocean County Fire Chief Brian Gabriel said in a news conference Friday.
Christine Hemingway, who was working inside Kohr Brothers Frozen Custard stand Thursday, said it seemed as if the smoke rose from beneath the boardwalk, WCAU-TV, Philadelphia, said.
"Our manager came running through the stand and said 'get out,' we go around the corner in front of Biscayne Candy and there was smoke coming up from the boardwalk," Hemingway said. "There was a little smoke and then all of a sudden, it got real thick and black."
Seaside Park Mayor Bob Matthies estimated the damage from the fire will be in the millions.
"The resiliency that was demonstrated before will be called upon again to rebuild this section of the town and the commerce on the boardwalk," said Matthies.