NORTH ADAMS — Firefighters battling the East Mountain Fire that spread across nearly 1,000 acres in the Clarksburg State Forest area have shifted to a patrol and “mop-up” operation.

The fire was 90 percent contained as of Tuesday evening, according to the Massachusetts Department of Fire Services.

As of Monday night, around 950 acres had burned, and the fire did not grow overnight between Monday and Tuesday, according to the department.

By Tuesday morning, the burden of the response had shifted to state and federal firefighters, who moved their focus to patrolling the perimeter of the fire and putting out hotspots.

“We’ll probably hold it in that status for the next couple of days because we don’t have any rain,” said Dave Celino, chief fire warden for Massachusetts Department of Conservation and Recreation. “We’ll end up with firefighters patrolling that fire until we get a significant change in the weather, with some precipitation, or we just finally don’t find any hotspots up there.”

Tuesday marked the fifth day of fighting the state’s largest wildland fire in more than two decades.

The fire started in Williamstown on Friday and had burned around 180 acres by Saturday morning. Over the weekend, it spread over the Appalachian Trail and across Clarksburg State Forest and reached some 800 acres total by Monday morning, making it the largest fire since 1999.

By Monday evening, the crews – a total of around 120 professional and volunteer firefighters from Massachusetts and Vermont – had brought the fire to 75 percent containment.

Celino said last year’s drought appears to have made the forest’s supply of leaves, logs and sticks abnormally dry, helping the fire spread quickly across steep, rugged terrain. In a normal year, he added, those dead plants would not be as available to burn until the mid or late summer, when conditions turn drier and hotter.

“What we’re seeing across the Northeast, especially where the drought really sped up last year, was that larger fuels ... never really recovered with moisture,” Celino said. “That means they become much more available to burn going into the next fire season, which we’re in now.”

Williamstown Fire Chief Craig Pedercini said that residents should expect to see smoke even after the fire is fully contained.

Francesca Paris can be reached at fparis@berkshireeagle.com and 510-207-2535.