A boy and girl, both about 15, crossed 169th Street at Parrish Avenue, heading north, when a vehicle traveling west hit them about 5:30 p.m., police said.
The vehicle had a green light at the time of the crash, police said.
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The girl was airlifted to a Chicago-area hospital and remains in critical condition. The boy was taken via ambulance to a Chicago hospital and is in intensive care but stable, police said.
No charges have been filed; the investigation remains ongoing.
"Our thoughts are with these young victims and their families as we work carefully to determine exactly what occurred," Hammond police said in a statement.