About 200 homes were forced to evacuate earlier in the day after the blaze erupted in the San Gabriel Mountains, authorities said.
The fire was sparked by equipment used by a gardener working the backyard, said Jennifer McLain, a city spokeswoman. Flames spread to the hillside behind the residence, scorching about 175 acres and sending a huge smoke cloud that could be seen across the San Gabriel Valley.
The fire did not threaten homes, but authorities ordered evacuations as a precaution. They began lifting evacuation orders late Saturday after firefighters contained 50 percent of the blaze, and winds subsided.
Full containment was expected Sunday.
Fire Chief Chris Donovan said dry, thick brush on steep terrain made the fire difficult to contain.
More than 200 firefighters, aided by water-dropping helicopters and an air tanker, were battling the blaze.
One firefighter suffered a heat-related injury.
As the Monrovia fire burned, a four-acre brush fire ignited across town near Interstate 405, forcing the closure of southbound lanes for at least an hour and causing traffic to back up for miles. That fire has been contained.
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