The Rim Fire became the fourth-largest wildfire in the history of California as it continued its steady burn Saturday, as smoke from the blaze for the first time darkened the sky over the Yosemite Valley, forcing many holiday vacationers indoors.

According to the statewide firefighting agency Cal Fire, 348 square miles have been scorched. At least 111 structures have been destroyed, but no one has been killed or seriously injured in the blaze, which is the largest wildfire burning in the country.

By last measure, the fire that has burned an area the size of Dallas was 40 percent contained with the help of more than 5,000 firefighters from state, federal and local agencies. Authorities said fresh firefighters were being brought in to replace tired crews, but that officials did not plan to reduce the number of personnel assigned to the fire.

The Labor Day weekend is historically a boon for Yosemite National Park as patrons flood the park before the school year begins and colder weather settles in. But the smoke has hampered the getaway vibe.

“I’m in Yosemite Valley right now, and I cannot see the cliffs around me,” said Kari Cobb, a park spokeswoman. “The wind has shifted, and smoke is impacting the entire park. We have been lucky until now.”

All the campgrounds in the valley still were full as of Saturday morning despite the thick blanket of smoke and the burning smell that permeated the area, Cobb said.

One vacationer staying in the historic Wawona Hotel, four miles from the park’s south entrance, called himself “a prisoner in the hotel” because of the invading smoke and partial road closures put in place earlier in the week.

The occupant, who asked not to be named because of privacy concerns, said it was clear as of Friday evening and that it “turned overnight,” with haze turning the sun orange and light ash visibly falling in the air.

Smoke from the blaze has drifted at least 2,500 miles, reaching Michigan, Indiana, Ohio and the Great Lakes. Overnight into Saturday, the Rim Fire surpassed the area burned by the 1932 Matilija Fire in Ventura County, which burned 220,000 acres.

At least 94 square miles of wilderness have burned in the northern section of Yosemite. Firefighting aircraft remained grounded because of low visibility caused by the smoke, U.S. Forest Service spokesman Mark Healey said.

Officials were concerned about a 150-acre spot fire that crossed a road and prompted an evacuation order for homes near the west entrance of Yosemite, Healey said.

Mandatory evacuations remain in effect south of Highway 120 and north Old Yosemite Road, while evacuations of areas surrounding Bull Creek Road to Grizzly Mountain have been lifted.

About 4,500 structures are still threatened east and west of the fire, which is expected to continue spreading east into the west side of Yosemite National Park east of Aspen Valley, according to a bulletin from Cal Fire and the Forest Service.

“Steep terrain and extreme fire behavior” are slowing firefighters’ progress, along with hot, dry weather forecasted for the coming week. Authorities said as a result they have become reliant on air tankers and drone planes to both fight and give them eyes on the flames. They have moved back the tentative fire containment date to Oct. 20.

The blaze’s cause is under investigation, though the Twain Harte fire chief suggested at an Aug. 23 community meeting that it likely stemmed from an illegal marijuana growing operation, which are common in secluded forest areas. The area where the fire started is roughly 10 miles west of the Yosemite National Park entrance on Highway 120 and 8 miles east of the town of Groveland — a rugged, steep expanse of dense wilderness.

Officials have not made any formal statement about the cause of the fire.

The Associated Press and Reuters contributed to this report.