Wed Jul 1, 2015 10:14pm BST

Investigators suspect human activity, rather than lightning, sparked a grassland fire that has destroyed at least 29 homes near the eastern slopes of the Cascades in central Washington, officials said on Wednesday.

The so-called Sleepy Hollow blaze, which has scorched nearly 3,000 acres since erupting Sunday on a drought-parched hillside west of the city of Wenatchee, continued to smolder as more than 300 firefighters gained ground on the flames.

As of Wednesday, crews had carved containment lines around nearly half the blaze's perimeter and were conducting "mop-up" operations in areas where flames had been substantially subdued, said Josie Williams, a spokeswoman for the fire command.

She said the tally of residences known to have been lost in the fire had risen to 29, five more than were counted on Tuesday.

Several businesses in downtown Wenatchee, a city of about 33,000 people, also were destroyed or badly damaged, including a recycling center, a farm chemical distributor and two fruit-packing warehouses, officials said.

Although the blaze followed a large number of lightning strikes in the region, investigators have since ruled out lightning as the fire's origin, deeming it to be human-caused instead, said Eileen Ervin, an emergency management official with the Chelan County Sheriff's Office.

There was no word yet on whether arson was suspected.

Mandatory evacuation orders for several hundred homes were lifted late on Tuesday, but an advisory telling residents to be ready to leave remained in effect for about 4,000 dwellings in the area, Williams said.

Washington has already experienced more than 300 wildfires so far this year, following the state's worst fire season on record in 2014.

"We've got a long, dry season ahead of us," Williams said. "We're hoping to get through the Fourth of July holiday."

The Sleepy Hollow blaze is one of dozens of large wildfires flaring across the western United States, including more than 20 in Alaska and 10 in Oregon.