The Pratt & Whitney engine that failed minutes into United Airlines flight 328 reportedly showed signs of metal fatigue.
Investigators with the National Transportation Safety Board (NTSB) revealed this finding during their first public briefing on Monday.
Saturday's in-flight incident led United Airlines to ground all of its Boeing 777s powered by Pratt and Whitney engines and prompted federal regulators to re-examine how often the engines should be inspected.
NTSB Chairman Robert Sumwalt said a preliminary on-scene investigation indicates damage consistent with metal fatigue on a fan blade in the plane's right engine.
Investigators said that blade broke free at the hub, likely hitting another that was broken mid-span.
A section of a blade will be examined at a Prattt & Whitney laboratory on Tuesday.
Investigators say they can determine for how long the blade was damaged before it failed.
The probe is expected to take more than a year.