The unidentified boy has been questioned over the fire in the Buckweed area, north of Los Angeles, which devoured swaths of Californian territory and spewed massive amounts of smoke into the atmosphere.

Alarms have also been raised about the longer-term effects of the pollution and the greenhouse gases produced by the fires, which ravaged 203,000 hectares of tinder-dry park and forest land.

San Diego County officials said 640,000 people, sharply higher than previous estimates of 500,000, had been displaced in the disaster.

Nearly 3,000 people remained in temporary shelters across the state as many people returned to find their homes reduced to piles of ash and rubble.

Hundreds of mental health experts have fanned out across the state to offer counselling and support to the thousands of people affected.

San Diego County officials have put the cost of property damage at more than $1US billion, although insurance industry analysts have said it may go as high as $US1.6 billion. © 2007 Australian Broadcasting Corporation