WASHINGTON—U.S. spy agencies now believe the preponderance of intelligence about last week’s crash of a Russian passenger plane over Egypt points to a terrorist bombing as the cause, U.S. officials said Sunday.

The evolving assessment, however, isn’t conclusive because it is based largely on intercepted communications and the accumulation of other fragmentary pieces of intelligence rather than forensic evidence, such as bomb fragments that have yet to be analyzed and could lead to a different conclusion, the officials stressed....