A tanker carrying a flammable liquid plunged off a flyover and exploded on top of other vehicles along a highway near Baltimore, killing at least four people.
"The people in the cars never had a chance," said Dwane Roberts, a truck driver who saw the tanker crash to the road below.
Gary McLhinney, police chief of the Maryland Transportation Authority, said the number of dead could rise in the crash.
At least three other vehicles also burned down to their frames during the massive, hour-long fire.
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Environmental officials believed the tanker was carrying a flammable petroleum product, but did not know what kind. Most of the product apparently burned away in the fire.