TULSA, Okla. — Betty Jo Shelby, the white Tulsa police officer charged with manslaughter in the shooting death of an unarmed black driver in September, testified on Monday that during the encounter, she relied on her training in how to deal with potentially dangerous suspects.

“I have all the indications that he has a gun,” Officer Shelby said under questioning by the prosecutor, Kevin Gray, of her response when the driver, Terence Crutcher, failed to follow her orders to stop moving toward his car. “I do not pull a Taser out, which is less lethal. I meet a gun with a gun.”

“In this situation, though,” Mr. Gray said, “you’re not meeting a gun with a gun. You’re meeting a guess about a gun with a gun.”

Responding to questions from her lawyer, Shannon McMurray, Officer Shelby testified that she thought Mr. Crutcher had a gun when he reached into a car window, and that she had been trained to shoot before he could take a shot.