The Fleet Reserve Building in Oak Harbor was damaged in a mid-morning fire Sunday, the Oak Harbor Fire Department reported.

Fire Chief Ray Merrill said the fire started inside the structure but the cause has not yet been determined.

An investigation is ongoing.

A witness reported seeing smoke coming from the roof of the building, which is in the southeast area of the city, at 10:38 a.m. Oak Harbor firefighters responded and arrived within four minutes, according to a press release from Merrill.

Fire crews entered the structure to find heavy smoke on the first floor with fire above a walk-in cooler in the kitchen area. The flames extended to the second floor and the attic.

The press release states that the command officer on scene requested mutual aid from Navy Region Northwest and North Whidbey Fire and Rescue. Each agency provided one engine company and a crew to assist with extinguishing the fire. Oak Harbor responded with one engine company, one ladder truck and two command units. WhidbeyHealth provided a paramedic unit.

“Due to the age of the building and the type of construction known as balloon frame construction there were no fire stops in the walls from the floor to the attic,” the press release states. “The fire extended from the first floor to the attic.”

It took the crews approximately 30 minutes to fully contain and extinguish the fire.

Merrill said the building sustained approximately $75,000 in damages, but the blaze was contained to to the kitchen, bar area, cooler and exterior walls. The main barn area was not damaged and the building remains structurally sound, he said.

The building was unoccupied at the time of the fire. One firefighter sustained a minor injury, the fire department reported.

The fire department responded to a minor fire at the Children’s Academy daycare the day before. Roofers had been using torches that ignited a small patch on the roof that smoldered overnight. Firefighters quickly responded and easily extinguished the fire.