Sergeant Lawrence Hutchins III was initially convicted of murder, larceny and making false statements over the killing of the civilian. His conviction was later overturned.
The official, who declined to be named because he was not authorised to speak to the media, said Hutchins was convicted again on Wednesday on three out of four counts, including murder.
Prosecutors have said Hutchins led a squad of Marines who planned a mission aimed at stopping militants' use of improvised bombs in the Iraqi village of Hamdania in the early morning hours of April 26, 2006.
When they could not find the suspected bomber, they went to a nearby house and took a disabled former police officer who was not a suspect, witnesses said in Hutchins' previous trial.
Hutchins and other Marines shot 52-year-old Hashim Ibrahim Awad, a father of 11 and grandfather of four, and placed an AK-47 and a shovel next to the corpse to suggest he had been planting a bomb, the witnesses said.
An attorney for Hutchins could not immediately be reached for comment.
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