Firefighters in the U.S. western state of California have successfully pushed back a wildfire threatening a historic observatory north of Los Angeles.

Officials said Thursday a firefighting force atop Mount Wilson managed to keep the flames from reaching the observatory and critical communication towers on the mountain.

The wildfire near Los Angeles has burned nearly 57,000 hectares of land, making it the largest wildfire in Los Angeles county's history.

By Thursday, safety officials had lifted all evacuation orders for residential areas, allowing people to return to their homes.

Officials say the fire, which has burned for more than a week, is now 28 percent contained.

Investigators are continuing to investigate the cause of the fire.  Deputy incident commander Carlton Joseph said Wednesday investigators believe the fire was caused by humans.

Carlton said investigators are still trying to determine if the blaze was started on purpose or by accident.  

The fire destroyed more than 60 homes in the area, forced thousands of people to evacuate, and led to the death of two firefighters.  The men died Sunday when their truck rolled off a step embankment.  

California Governor Arnold Schwarzenegger visited firefighters at a Los Angeles base camp Wednesday.  The governor said of the eight wildfires currently burning across the state of California, three have been contained. 

Earlier this week, fire commander Mike Dietrich said it could take another two weeks to contain the fire. 

Some information for this report was provided by AFP, AP and Bloomberg.