Firefighters responded around 12:30 p.m. to an area just north of San Dimas, where the fire had burned about an acre, officials said. Firefighters were able to stop the fire - which grew to about four acres - about an hour later.
"The threat to the National Forest/San Gabriel Mountains National Monument has been reduced," read an update from officials posted on X.
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No injuries have been reported.
The cause of the fire remains under investigation.
This is a developing story. This article will continue to be updated as more information becomes available.