RIVERSIDE (CBSLA.com) — Fire crews in Riverside on Saturday evening were fighting a brush fire that broke out near the Santa Ana River bottom.

The fire was first reported at about 2:30 p.m. near Beacon Way and Ladera Lane, just outside Fairmount Park.

READ MORE: Grandmother, 4 Children Shot And Killed In Lancaster Home; Father Arrested

By 6:30 p.m., the fire had grown to 35-40 acres, authorities said.

The fire was 50 percent contained and was threatening some structures, firefighters said.

READ MORE: Kim Fields Hopes 'Adventures In Christmasing' Inspires People To Come Out Of Their Comfort Zone

Units from Cal Fire and Corona Fire Department were on the scene, providing protection to nearby houses.

No evacuations had been ordered initiated, and no injuries were reported, fire crews said.

The cause of the fire was under investigation.

MORE NEWS: Super Lotto Ticket Worth $38 Million Sold At Santa Clarita Gas Station

(©2015 CBS Local Media, a division of CBS Radio Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. Wire services contributed to this report.)