A small plane crashed into a commercial building in California on Thursday, killing at least two people and injuring 18 others, police said.

The crash occurred early in the afternoon near Fullerton Municipal Airport, 25 miles (40 kilometres) southeast of Los Angeles. The cause remains unknown.

“There are two confirmed fatalities,” Fullerton police said on X.

Active Investigation Update:

There are two confirmed fatalities, ten individuals were transported to area hospitals for treatment and eight individuals were treated and released on scene. The investigation is on going.

Additionally, 10 people were hospitalized and eight others were treated at the scene.

Investigators do not yet know if the victims were plane passengers or if they were workers at the building where it crashed, a police officer told local station KTLA.

Television footage showed a gaping hole in the roof of the building, with smoke pouring out.

“All we hear is like a loud noise, boom, and that’s it. Then we started running out” of the building, Jerome Cruz, one of the workers who witnessed the scene, told CBS News.

The plane was a single-engine Van’s RV-10, a small model with four seats, according to the Federal Aviation Administration, which has opened an investigation.

Another crash near Fullerton Airport injured two people in November.