Fire officials are investigating the cause of a Sunday brush fire on Waipio Valley Trail in Waimanu Valley.
Twelve units responded to the 8:44 a.m. alarm with the first unit arriving on scene 19 minutes later.
A county helicopter provided water drops and firefighters prevented hikers from approaching the danger area, a two-acre brush fire below the forest canopy.
State Division of Forestry and Wildlife responded with a ground crew later in the day to help contain the fire, which according to a written Fire Department statement, had expanded to four acres by nightfall.
The trail remained open, and the fire was neither extinguished nor contained as of Sunday night.