Four days went by before officials acted on satellite data showing Malaysia Airlines Flight 370 flew for several hours away from the area being covered by a massive international search, people familiar with the matter said—a delay from which investigators are still working to recover.

The satellite's operator, Britain's Inmarsat PLC, on March 11 turned over to a partner company its data analysis and other documents indicating that the plane wasn't anywhere near the areas on either side of Malaysia where more countries and...