The 153-acre grass fire that caused evacuations and closed a major highway Monday was likely an arson, officials announced Tuesday.

The blaze, dubbed the Oak fire, was reported around 2:30 p.m. burning on the hogback along Colorado 470 near South Kipling Parkway. It grew to 150 acres in around two hours, but did not damage any buildings.

As of 5:45 p.m. Tuesday, firefighters had contained 70% of the fire, West Metro Fire Rescue said. Work continued throughout the day as 25 firefighters established a perimeter and mopped up the scene.

Officials said they do not know exactly how the fire started, but it was set west of the Westerly Apartment complex in an open space with several trails and heavy foot traffic.

The Jefferson County Sheriff’s Office said it is investigating the fire as an arson and they have identified a suspect, though no arrests or suspect information have been announced.

On Monday, the apartment complex and the Ken Caryl Valley were evacuated as the blaze threatened homes. Both evacuations were lifted by the end of the day.

C-470 was closed from West Ken Caryl Avenue to South Kipling Parkway for several hours and Jefferson and Clear Creek counties were put on accident alert due to the fire, according to the Colorado State Patrol.

Two strike teams and a single-engine air tanker were sent to fight the blaze Monday, West Metro Fire Rescue said. As firefighters worked to extinguish the blaze on Tuesday, residents were expected to see more smoke in the area.

In addition to West Metro Fire Rescue, the Arvada Fire Protection District, Inter-Canyon Fire Protection District, Foothills Fire Rescue and Colorado Division of Fire Prevention and Control worked to contain the blaze on Tuesday.