She urged the FBI to investigate the murder and called on the US government to determine whether Saudi Crown Prince Mohammed bin Salman could be held responsible under the Global Magnitsky Human Rights Accountability Act.
Before his death, he had been living in the United States, near Washington, D.C. He wrote for the Washington Post and other news media.
In a fresh report, the UN's special rapporteur on extrajudicial, summary or arbitrary executions, Agnes Callamard, said she had "determined that there is credible evidence, warranting further investigation of high-level Saudi Officials' individual liability, including the Crown Prince's".
After the killing, some Western executives pulled out of an investment forum in Riyadh.
Saudi Arabia had been trying to get him to return from the United States, where he worked as a journalist for the Washington Post, but he feared for his safety. The UN report will serve as fodder for his multiple enemies both at home and in the region.
His dismembered body has not been found.
Jamal Khashoggi, left, and Crown Prince Mohammed bin Salman.
There is "credible evidence" linking Saudi Arabia's Crown Prince to the killing of Saudi journalist Jamal Khashoggi last October, a report by an independent United Nations rights expert found on Wednesday, reported AFP.
"The report. contains clear contradictions and unfounded allegations, casting doubt on its credibility".
The Gulf kingdom has put 11 unidentified people on trial behind closed doors for Mr Khashoggi's murder and is seeking the death penalty for five of them. She received no answer to her request to travel to Saudi Arabia. But UN spokesman Stephane Dujarric said Guterres could only do so with a mandate from "a competent intergovernmental body".
The U.N. Human Rights Council, where Saudi Arabia is among the 47 member states, opens a three-week session on Monday. Saudi Arabia has regularly maintained its stance that denied the involvement of the prince.
The US's spy agency has reportedly blamed MBS for Khashoggi's murder, while US senators have passed a resolution assigning responsibility for the killing to the crown prince. "The only conclusion made is that there is credible evidence meriting further investigation, by a proper authority, as to whether the threshold of criminal responsibility has been met". She and her Arabic translator were allowed to listen to about 45 minutes of audio, though she believes Turkish intelligence has about seven hours of recordings.
Turkish President Tayyip Erdogan said those responsible for the murder will pay the price. The U.N. report backs up that finding, pointing to the magnitude of coordination and resources behind the murder plot as evidence that the highest levels of Saudi leadership must have known about it.
A few minutes before Khashoggi arrived, Salah al-Tubaigy, an Interior Ministry forensics doctor who would dismember the body, said he hoped his job would "be easy", according to the report.
"Joints will be separated", Tubaigy said, according to the report. It is not a problem.
Sen. Lindsey Graham, R-S.C., a close ally of the president, said in a statement at the time, "While I understand that Saudi Arabia is a strategic ally, the behavior of Mohammed bin Salman can not be ignored".
Callamard welcomes the fact that the USA, and then subsequently Canada, the UK, France, Germany and the European Union, took the initiative to sanction individuals for their role in Khashoggi's murder. "We will wrap each of them".
The report comes as damage to the crown prince's reputation had begun to fade, with countries and companies resuming business with the uber-wealthy kingdom.
"No conclusion is made as to guilt", Callamard wrote of the two men. The special rapporteur's report concludes: "Assessments of the recordings by intelligence officers in Turkey and other countries suggest that Mr Khashoggi could have been injected with a sedative and then suffocated using a plastic bag".
Reacting to the report, Human Rights Watch criticised what it called the Saudi "intolerance of dissent and climate of impunity" that enabled the murder.
Callamard said current sanctions do not go far enough and "fail to address the central questions of command and of senior leadership's responsibilities for and associated with the execution".