Prince Mohammed bin Salman speaks with a Saudi prince in Riyadh. File picture: Hassan Ammar/AP

Geneva - Saudi Crown Prince Mohammed bin Salman and other senior Saudi officials should be investigated over the killing of dissident journalist Jamal Khashoggi, a UN rights expert has concluded on Wednesday, citing "credible evidence."

Khashoggi, a Washington Post columnist and critic of the crown prince, was allegedly murdered and dismembered by Saudi operatives in the Saudi consulate in Istanbul in early October.

Agnes Callamard, the UN special rapporteur on extrajudicial killings, said in her report that the case must be viewed in the context of detentions and torture of other journalists and activists in recent years.

"At a bare minimum, [the] Crown Prince condoned this behavior and allowed the repetition and escalation of these crimes," Callamard wrote about the royal who is seen as the de-facto ruler of his country.

"The Crown Prince willingly took the risk that other crimes, such as the killing of Mr Khashoggi, would be committed, whether or not he directly ordered the specific crime," she added.

Khashoggi was afraid of the prince and his powerful status, she wrote. Various Saudi emissaries tried to convince Khashoggi to return home, but he feared what would happen to him if he did so.

Callamard said every expert she consulted for her report said that Prince Mohammed must at least have been aware of the Khashoggi operation, as it required significant government coordination and resources.

She added that the crown prince revealed that he was closely following the matter in a television interview that he gave shortly after the killing, but before the crime was confirmed.

The destruction of evidence could also not have taken place without his knowledge, she concluded.

The search for justice in this case is not primarily about "finding a smoking gun and the person holding it," Callamard wrote.

An independent UN human rights expert investigating the killing of Saudi journalist Jamal Khashoggi is recommending an investigation into the possible role of Saudi Crown Prince Mohammed bin Salman, citing "credible evidence." Picture: Hasan Jamali/AP

Rather, the search is about those who "have abused, or failed to fulfil, the responsibilities of their positions of authority."

Callamard said that countries should claim universal jurisdiction in this case, meaning that suspects could be prosecuted and tried outside Turkey or Saudi Arabia.

In addition, she called on UN Secretary General Antonio Guterres to set up a "follow-up criminal investigation" of the perpetrators and to come up with a mechanism for a tribunal.

Saudi Arabia has started court hearings against 11 defendants, five of them facing the death penalty.

Callamard concluded that neither Saudi Arabia nor Turkey has followed up the case in line with international standards so far.

A joint crime scene investigation by experts from both countries only took place two weeks after the incident, and Turkish officials were only allowed six hours to do their work.

No blood traces were found, pointing to two possibilities, according to Callamard: Either the killing was planned and measures were taken to prevent the spread of fluids; or the crime scene was scrubbed to remove evidence.

Turkish authorities failed to search the residence of the Saudi consul in Istanbul soon enough, and they did not interview this official, Callamard criticized.

The UN rights investigator will present her report at a regular UN Human Rights Council session that starts next week in Geneva.