An investigation into a one-alarm fire that started at Skyline Elementary School in Daly City on Sunday night is underway, according to the North County Fire Authority.

Daly City fire official believes fire was arson.

Firefighters responded to the blaze, which started in a portable classroom at 55 Christen Ave., around 10:30 p.m., fire officials said.

Using hose lines, crews were able to contain the flames to the building and extinguish the fire. Firefighters worked on the scene until about midnight completing overhaul, fire officials said.

No one was injured in the fire, but its cause and the estimated damage to the school has not yet been determined.