Investigators told WHBQ that when Whitaker got into the back of a Pontiac sedan, they robbed him.
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Witnesses at the scene said Whitaker was shot as he ran away from the car.
Police said Whitaker was dropped off at the fire station on Lamar with a gunshot wound, then taken to Regional One, where he died.
Investigators reportedly found Spencer in the 900 block of Hale in Whitehaven. She waived her rights and told police she was with two males at the time of the crime, authorities said.
Spencer reportedly told police she posted the ad to sell the gun. Police said Spencer informed them that one of the men shot and robbed Whitaker while he ran away.
Spencer has been transported to Jail East for charges.