Its passengers were attending a concert on the grounds, and no one was on the bus at the time of the fire, Katonah Fire Chief James McHugh said.
Firefighters sprayed foam on the flames coming out of the bus to knock down the blaze, McHugh said. The bus was from Alert Coach Lines, a company based in Ronkonkoma.
At about 4 p.m., the bus driver and Katonah firefighters were retrieving passengers' belongings and transferring them off the bus.
Bedford police also responded to the scene.
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