The engine failed with a "loud bang" four minutes after takeoff from Denver.
The National Transportation Safety Board have been looking at damage to a fan blade.
The chairman of the U.S. air accident investigator told reporters it remains unclear whether the incident is consistent with an engine failure on a different Hawaiian-bound United flight in February 2018.
That failure was attributed to a fatigue fracture in a fan blade.
The engine that failed shed parts over a Denver suburb.
The Pratt & Whitney engine, the PW4000, is used on 128 planes, or less than 10% of the global fleet.
In another incident on Japan Airlines in December 2020 the country's transport safety board reported it found two damaged fan blades, one with a metal fatigue crack on the same engine. That investigation is ongoing.
The focus is more on engine maker Pratt and analysts expect little financial impact on Boeing.
But the PW4000 issues are a fresh headache for the planemaker which saw its flagship 737 MAX grounded for nearly two years after two deadly crashes.
Investigators will continue to examine the United engine's fan blade on Tuesday (February 23).