The investigation into the fire that destroyed Fort Clatsop three weeks ago has concluded that a fire in a fireplace was the cause.

A joint report issued Thursday determined that the fire, which broke out the night of Oct. 3, was accidental.

"It's not a surprise," said Chip Jenkins, superintendent of the Lewis and Clark National Historical Park.

The fire largely destroyed the 50-year-old replica. Investigators from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, assisted by local police and fire agencies, pinpointed the starting point of the blaze as Room No. 2 in the "Enlisted Mens' Quarters" side of the fort.

A fire had been lit in the fireplace of the room that day as part of the interpretive programs held in the fort, which hosted a number of schoolchildren.

"From my perspective, the most important thing is taking the lessons learned and incorporating them into what we do next," Jenkins said.

Crews have cleared the site of debris, and will next be removing the gravel that lay under the wood floors. The site, which was put off-limits during the demolition work, is again be open to the public.

Next week, National Park Service archeologists are set to begin a probe of the ground beneath the replica, the first comprehensive look at the site since the fort's construction 50 years ago.