The fire started Thursday, the DNR said, and a boat was seen quickly leaving the area at the time of the ignition.
Officials are asking any one that saw suspicious activity near Lake Chelan or has any information about the investigation to call lead investigator Alan Lawson at 509-925-8510 or email@example.com.
The wildfire has expanded to more than 1,550 acres, the Washington Incident Management Team said.
Authorities have contained 10 percent of the fire, KCPQ-TV, Seattle, reported.
A level-one evacuation notice, which warns residents of the danger in the area, has been issued for two nearby communities.
Residents who have special needs or are sensitive to smoke should evacuate, officials said.
Fire crews are keeping watch on shifting wind patterns, which could impact the fire.