A flare gun was found near the start of the 11-acre Gilbert and Castlewood fire that prompted evacuations Wednesday, officials said.
Firefighters had the blaze contained to the northeast corner of Gilbert Street and Castlewood Drive after it was reported shortly before 8 p.m., according to the Fullerton Police Department.
By 9:42 p.m., fire officials said structures were no longer threatened.
Residents who were told to leave their homes were allowed to return about 10:30 p.m. Thursday.
Roadways that were blocked during the blaze were also reopened.
Fullerton police officials did not elaborate on the flare gun that was found, but indicated that arson investigators were at the scene working to determine the cause of the blaze.
In addition, firefighters were expected to remain at the scene of the blaze to ensure that it is fully out and no flare ups cause problems.
Aerial video from Sky5 shows flames lighting up a patch of thick vegetation nestled between homes. Thick smoke can be seen billowing over the area as crews battled the blaze.
A palm tree in a nearby home's front yard caught fire and firefighters were working to douse it with a water hose, video showed.
A family could be seen evacuating the home as embers flew from the tree overhead.
Another brush fire broke out earlier in Fullerton Wednesday near the Brea Dam. Crews were able to subdue the blaze and stop flames from spreading.
No further details were immediately available.