A fire burning in the Santa Catalina Mountains has grown to about 200 acres overnight.

The Prison Fire, which started around 6 p.m. Sunday just west of milepost 8 on Catalina Highway, is burning in an area that was a former WWII prison camp in the Molino Basin, said Gerry Perry, a fire information officer with the Tucson Interagency Dispatch Center.

The fire was likely started by a lightning strike and is burning in light fuels that have grown since the Aspen Fire, Perry said.

The fire is about 5 percent contained and no structures are being threatened, Perry said.

Two crews, an engine, two water tenders, a helicopter and 50 firefighters are battling the blaze.

Catalina Highway remains open but motorists are asked to avoid the area to allow crews to work on the fire and avoid heavy traffic.

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