(Adds details on fire)
SAN FRANCISCO, Sept 5 (Reuters) - A massive California wildfire that has blackened a swath of Yosemite National Park and charred an area of land larger than the city of Dallas was sparked by a hunter who let an illegal fire get out of control, fire managers said on Thursday.
The so-called Rim Fire, which broke out on Aug. 17 in the Stanislaus National Forest and is now 80 percent contained, has charred more than 237,000 acres, making it the fourth largest in modern state history.
A statement from fire managers working with the U.S. Forest Service said investigators had determined the fire began "when a hunter allowed an illegal fire to escape." They did not release the hunter's name and said no arrests had been made.
The news came a day after officials said they had ruled out the possibility that an illegal marijuana growing operation started the fire, which is still expected to burn for several more weeks as crews worked to protect Yosemite, one of the nation's top tourist destinations.
So far, the Rim Fire has scorched tens of thousands of acres inside Yosemite, mostly in less-visited back country areas. Several campgrounds have been closed, along with parts of Highway 120 leading to one of its two west entrances.
The most popular portions of the park remain open, including the scenic Yosemite Valley area famed for its towering granite rock formations, waterfalls, meadows and pine forests.
The fire has destroyed dozens of homes and cabins in the region, but no serious injuries have been reported. (Reporting by Laila Kearney; Writing by Cynthia Johnston; Editing by Bernard Orr)