(New throughout, updates details of investigation, fire)
By Laila Kearney
SAN FRANCISCO, Sept 5 (Reuters) - A California wildfire that has spread through a swath of Yosemite National Park and burned an area larger than Dallas, Texas, was sparked when a hunter's illegal fire got out of control, fire managers said on Thursday.
No arrests have been made yet, the Forest Service and the local district attorney's office said in a statement. They did not name the hunter or say how investigators had determined the cause of the fire. The Tuolumne County District Attorney's office declined to comment because the investigation is active.
Knocking down earlier speculation by a local fire official, investigators said they had ruled out any connection between the fire and the illegal cultivation of marijuana.
Officials said the fire is 80 percent contained, but they expect it to burn for several more weeks as crews work to protect Yosemite, a popular tourist destination.
The fire has burned more than 237,000 acres, making it the fourth largest fire in state history, according to the California Department of Forestry and Fire Detection. It has destroyed dozens of homes and cabins in the region, but no serious injuries have been reported.
Almost 2,500 homes are currently threatened by blaze that officials have dubbed the Rim Fire, after a popular local lookout area called Rim of the World.
At the height of the blaze, some 5,000 firefighters were enlisted to battle the largest U.S. wildfire this year. Battling the Rim Fire has cost $81 million to date.
The fire broke out on Aug. 17 at Jawbone Ridge in remote Stanislaus National Forest. Campers there normally need fire permits in the summer. But more than a week before the fire broke out, the Forest Service had ordered temporary fire restrictions banning all campfires in the area.
Since spreading to Yosemite, the blaze has scorched tens of thousands of acres inside the park, mostly in less-visited back country areas.
Several campgrounds at the national park have been closed, but the most popular portions have remained open, including the scenic Yosemite Valley area famed for its towering granite rock formations, waterfalls, meadows and pine forests. (Reporting by Laila Kearney; Writing by Cynthia Johnston; Editing by Bernard Orr and David Gregorio)