WARRENTON, Ore. — Less than five weeks away from the bicentennial celebration of the Lewis and Clark expedition's arrival at the Pacific Ocean, the replica fort that commemorates their rain-soaked coastal stay has been destroyed by fire.

The blaze erupted Monday evening in the log cabin and stockade structure inside Lewis and Clark National Park. By the time the first responders arrived after 10 p.m., the flames were shooting more than 20 feet high.

Investigators have not determined the cause of the blaze. But firefighters spotted a full-size Chevy pickup leaving a parking lot, prompting Park Service officials yesterday to label it a "vehicle of interest."

Fort Clatsop draws some 270,000 visitors annually. It was built 50 years ago in a community effort that included more than 400 logs donated by a timber company and the careful craftsmanship of the Finnish Brotherhood of Astoria. It is the major draw of this small national park tucked into coastal groves of Sitka spruce, fir and hemlock.

"I'm just stunned," said Jan Mitchell, chairwoman of Destination: The Pacific, which is organizing the Nov. 11-14 commemoration at more than a half-dozen sites — including Fort Clatsop — along the northwest Oregon and southwest Washington coast. The events are expected to draw tens of thousands of visitors.

"Of all the things we could worry about, this was not something we ever could have anticipated," Mitchell said.

Mitchell and Park Service officials said they were not aware of any threats that had been made against the commemoration events. They described the fort as a cherished local landmark.

Early yesterday afternoon, as federal and state investigators inspected the charred remains, Mitchell, Park Service officials and Oregon Gov. Ted Kulongoski vowed that the fort would be rebuilt.

They said the fire would have no effect on the November commemoration.

Local companies offered to donate timber, the Oregon National Guard and private citizens offered labor, and U.S. Interior Secretary Gale Norton had directed the Park Service to make "all resources available" to help rebuild the fort, according to Chip Jenkins, the park superintendent.

Construction of the original fort began in December 1805. After an 18-month journey from the mouth of the Missouri River to the Pacific, William Clark, Meriwether Lewis and their Corps of Discovery were preparing to hunker down for the winter.

They settled along a small river near present-day Warrenton, hunting elk for meat and felling timber to form two sets of connected cabins that faced each other across a small central parade ground. Cold and wet weather, rotten clothes and fleas made for a miserable start to the winter.

But most of the compound was completed by Christmas Eve 1805, according to the book "Fort Clatsop, The Story Behind the Scenery."

By the 1950s, the area of the old fort was a wasteland, littered with trash and described by The Oregonian as being in "deplorable" condition. The Astoria Junior Chamber of Commerce adopted the site and organized a cleanup. The replica of the fort was built two years later.

Like the original, the replica consisted of four rooms in one set of adjoined cabins and three rooms in other cabins. It is located in an area believed close to the first fort. But it was far from an exact copy, using peeled logs and some Scandinavian-style construction, according to a Fort Clatsop history compiled by the Park Service.

The blaze was reported by someone who saw the flames from several miles away. Initially, amid the evening fog, there was confusion about the location of the fire.

A locked gate blocked the way to two visitor parking lots near Fort Clatsop. But the "vehicle of interest" was spotted exiting one of the unblocked employee parking lots.

"I got there and they left, I don't know if they had anything to do with it," said Ian O'Connor, chief of the Lewis and Clark Volunteer Fire Department, who reported the vehicle to a sheriff's dispatcher. Oregon State Police are asking anyone with information about the vehicle to contact them.

When the firefighters arrived, the fort was already fully ablaze. One set of connected cabins had collapsed, according to firefighters.

The fort is deemed a total loss, and a debate already is under way about how it should be rebuilt. Some would like it more true to the crude methods available to the Corps of Discovery, while many in the community took pride in the craftsmanship of the sturdier replica with its uniformly milled, specially treated logs.

"I know there's going to be a lot of discussion," said Scott Stonum, a Park Service official. "But it's not just about the structure — it's the story behind it that's important."

Even yesterday, the story of that pivotal exploration in American history was still being told — and retold — as schoolchildren arrived by bus in the park. Though yellow police tape marked the fort off-limits, the students sat underneath a pavilion to listen to costumed interpreters — holding tanned skins and fur — talk about the Corps of Discovery.

"We've been building up to this since the first day of school," said Betsy Carroll, a fifth-grade teacher at Island School on Bainbridge Island. "The kids really wanted to see the captain's quarters, but they're still excited to be here."

Hal Bernton: 206-464-2581 or hbernton@seattletimes.com