TULSA, Okla. — The white police officer who fatally shot an unarmed black driver here last week as he stood outside his vehicle overreacted during a confrontation captured on video and was charged on Thursday with first-degree manslaughter, the authorities said.

According to court documents, the officer, Betty Jo Shelby, 42, was overcome with fear that the man, Terence Crutcher, 40, who was not responding to her commands and was walking away from her with his hands up, was going to kill her.

An investigator with the Tulsa County district attorney’s office said in an affidavit that Officer Shelby became “emotionally involved to the point that she overreacted” and fired her weapon even though she “was not able to see any weapons or bulges indicating” that Mr. Crutcher had a gun.

Prosecutors have charged the officer with committing manslaughter “in the heat of passion.” Oklahoma law defines such passion as a strong emotion, such as fear or anger, that exists to such a degree in a defendant that it affects “the ability to reason and render the mind incapable of cool reflection.” Those found guilty of first-degree manslaughter face a sentence of no fewer than four years in prison.