The Uber Technologies Inc. self-driving car involved in a fatal crash in Arizona identified an object on the road six seconds before impact and didn’t determine the need for emergency braking until nearly five seconds later, U.S. safety investigators said Thursday.

The calculation ultimately wasn’t acted on as the car’s built-in automatic emergency braking system was disabled and the safety operator responsible for performing such a maneuver didn’t apply the brakes until after the collision, the National Transportation Safety...