WATERTOWN (CBS) – Four businesses on one block were gutted by a three-alarm fire in Watertown late Tuesday night.

The flames broke out in a commercial building on Main Street. A passing truck driver spotted the fire and called 911.

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It took firefighters more than two hours to put the fire out.

The roof collapsed after an explosion that sent one firefighter to the hospital with a shoulder injury.

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The building housed a barber shop and insurance company offices.

There’s no word yet on the cause of the fire.

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Damage is estimated in the hundreds of thousands of dollars.