Khashoggi was a resident of the US and worked for the Washington Post newspaper, giving his case a strong local angle for lawmakers and the media.
"Today’s verdicts were an important step in holding those responsible for the terrible crime accountable," the senior official said, adding that the trial is part of "a judicial process that continues."
"I know that there are some who would say that this hasn’t touched everybody responsible in the kingdom," the official said, acknowledging critics.
The United Nations' special rapporteur on extrajudicial killings, Agnes Callamard, described the trial as a "mockery" that failed to identify the masterminds or "those who incited, allowed or turned a blind eye to the murder, such as the Crown Prince."
The US official noted that Washington sanctioned Saudis alleged to have been involved in the murder, which took place at the Saudi consulate in Istanbul last year.
The official also tried to deflect attention away from Saudi Arabia, attempting to point out human rights abuses in China and Iran.
Five people were sentenced to death over the murder following a secretive year-long trial.
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