MOHAMMED AL-SHAIKH/AFP/Getty Images(BEIRUT) -- The killing of Jamal Khashoggi was a crime perpetrated at the highest levels of the Saudi Arabian power structure and requires further investigation of ranking Saudi officials, including Saudi Crown Prince Mohammed bin Salman, according to a new report published by a special U.N. investigator.

Khashoggi, 59, was a Washington Post columnist and a Saudi Arabian citizen who had legal residency in the United States.

Human rights legal advocate and United Nations special investigator Agnes Callamard published her conclusions Wednesday following a months-long examination into the details of Khashoggi's death at the Saudi consulate in Istanbul on Oct. 2.

Khashoggi's execution constituted an "extrajudicial killing for which the State of the Kingdom of Saudi Arabia is responsible," the report stated.

Saudi Arabia's minister of state for foreign affairs, Adel al-Jubeir, rejected the content of the U.N. report as "nothing new." He added in a tweet that the report "contains clear contradictions and baseless allegations which challenge its credibility."

Al-Jubeir also said the 101-page report only reiterated what has already been reported in the media. "We affirm that the sovereignty of the Kingdom and the jurisdiction of its judicial institutions over this issue is not compromised," he said.

Although stopping short of lodging a formal accusation against the Saudi crown prince for ordering the murder, the report recommended there was enough "credible evidence" to justify further investigation.

Bin Salman, Saudi Arabia's day-to-day ruler, shares power with his elderly father, the king, and wields a great amount of authority in the kingdom, demanding strict loyalty and fealty from all his top-level executives.

The report detailed at length the power structure in the kingdom, which in almost all cases emanates downward from the office of the crown prince. The report concluded that any campaign against dissidents and political opponents would not be possible without the crown prince's "agreement of acquiescence."

The large amount of government coordination, resources and finances required to fund and expedite the 15-member team believed to have been dispatched to Turkey to carry out the execution was significant, according to the U.N.'s findings. "Every expert consulted finds it inconceivable that an operation of this scale could be implemented without the Crown Prince being aware, at a minimum, that some sort of mission of a criminal nature, directed at Mr. Khashoggi, was being launched," the report said.

"Taking accountability seriously means that the Saudi Arabia government must accept State responsibility for the execution," Callamard wrote.

Callamard also found serious flaws in the Saudi investigation in the days immediately following the murder, citing "credible evidence pointing to the crime scenes having been thoroughly, even forensically, cleaned." The report went on to allege that "the Saudi investigation was not conducted in good faith, and that it may amount to obstructing justice."

Despite a wide search by Turkish investigators in the weeks and months following the murder, Khashoggi's body has never been found.

Khashoggi's abduction and murder by a team of Saudi operatives provoked global outrage and tarnished the image of the crown prince, who had been celebrated as a moderate reformer who championed modernizing Saudi Arabia's finances and granting women the right to drive.

Callamard denounced the lack of transparency at the kingdom's secretive hearings for 11 suspects accused in the murder, and called on Saudi authorities to reveal the defendants' names, the charges against them, and the fate of 10 others initially arrested.

The Trump administration is pressing Saudi Arabia to show "tangible progress" toward holding Khashoggi's killer's accountable, and wants them to do so before the one-year anniversary of his murder, a senior administration official said one week ago.

In the wake of her findings, Callamard endorsed a course of action for the United States. It includes opening an FBI investigation into the execution of Khashoggi and pursuing criminal prosecutions in the U.S. as appropriate. She also implored Congress to hold hearings to determine the responsibility of high-level Saudi officials, including the crown prince, and to demand access to underlying classified materials.

"To the greatest extent possible consistent with national security," the report recommended, the United States should "declassify and release to the public all materials relating to the murder of Mr. Khashoggi, including all intercepts."

"Callamard's report underscores that there will be no justice for Jamal Khashoggi unless Congress steps up," Rob Berschinski, a former Obama-era State Department official and Senior Vice President for Policy for the advocacy group Human Rights First told ABC News. "Saudi leaders have made it clear that they intend to get away with murder. President Trump has made it clear that he values arms sales over the killing and dismemberment of a U.S. resident. Congress must make it clear that it will not let this stand."

A failure by Washington to respond to the U.N. recommendations would "signal a green light to autocrats around the world that they can murder dissidents without repercussion, even if those dissidents seek shelter in the United States." Berschinski said. "The future safety of Americans, and that of brave activists around the world, depends on it."

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