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A week-old wildfire in Northern California continued to pose a threat to buildings Tuesday while firefighters increased containment of two other big forest fires in the region.

The Salt Fire in the Shasta Lake area north of Redding grew to more than 19 square miles and was just 20% contained, Shasta-Trinity National Forest officials said.

Priorities for the 1,400 firefighters included providing structure defense in the northwestern and southeastern areas of the fire, which has destroyed 41 buildings, including 27 homes.

A community meeting was told Monday that investigators determined the fire was ignited around 1:30 p.m. June 30 by unspecified hot materials that separated from a northbound vehicle on Interstate 5 and landed in very dry vegetation. The vehicle remained unidentified.

Residents were also told that the firefighting force was bolstered by resources released from the two other big fires to the north.

The Lava Fire north of the city of Weed was 72% contained after burning more than 39 square miles. The 16.5-square-mile Tennant Fire was 57% contained.

In Southern California, a 1.5-square-mile fire near Interstate 5 in northern Los Angeles County was 65% contained.