A woman was fatally injured today when she was struck by a tractor-trailer on Interstate 495 in Haverhill this morning, State Police said.

Troopers responded to the crash at around 9 a.m., police said. Police closed the left lane of the busy highway to assess the scene. The driver of the truck remained on scene.

State Police are investigating the accident and trying to determine why the woman was walking on the highway, which is prohibited.

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No further information was immediately available.