UN human rights chief, Michelle Bachelet has called for international experts to be involved in an independent investigation into the killing of Saudi journalist, Jamal Khashoggi, with access to evidence and witnesses.

In a statement on Tuesday, Bachelet also urged Saudi Arabia to reveal the whereabouts of Khashoggi’s body, saying that a forensic examination and autopsy were crucial elements in an investigation into the “shockingly brazen crime” carried out in the Saudi consulate in Istanbul on Oct 2.

At a memorial service for Khashoggi in London, his fiancee, Hatice Cengiz, said there must be no cover-up, and called for western countries to demand justice.

Cengiz said she had been disappointed by some countries’ response to his death, singling out Donald Trump for “paving the way for a cover-up”.

Khashoggi’s fiancee said she believed that the Saudi government knew the whereabouts of his body, and that her life was turned upside down “on the horrible day that Jamal entered the Saudi consulate”.

“If only I had known there was such an evil bloodthirsty crew I would have done everything to stop him entering that building,” she said.

“We never imagined such a level of barbarity and evil.”

Cengiz described Khashoggi as a “patriot who felt it his duty to become the voice of the voiceless”.

She added: “He wrote what he believed and only wrote what he knew was right, and not what would satisfy the powerful.”

Western powers believe Erdoğan is still holding back evidence in a bid to back Prince Mohammed into a corner.

Turkey has yet to release an audio tape which was allegedly a recording of the killing of Khashoggi, or evidence to show that the 15 Saudi operatives inside the Saudi consulate in Istanbul made four phone calls to the crown prince’s office, as pro-government Turkish media have claimed, based on leaked information.

Khashoggi’s remains have not been found but cars from the Saudi consulate went to the consul general’s residence, with one vehicle continuing to woodland on the edge of Istanbul.

Pro-government Turkish media have published new photos of cars owned by the Saudi diplomatic mission being cleaned before police were allowed to search them.

A driver who drove some of the Saudi team to the airport after Khashoggi’s murder told A Haber television that the men had appeared relaxed and cheerful, smoking and drinking alcohol on the way home.

Khashoggi’s death has caused a major diplomatic crisis for Saudi Arabia and has cast an unwelcome spotlight on bin Salman, whom many believe must have been aware of an operation that allegedly involved several members of his personal guard and trusted senior officials.

Turkish prosecutors have prepared an extradition request for 18 suspects arrested by Riyadh as part of the Saudi investigation, including the 15 who were at the consulate, although the Saudi foreign minister insisted on Saturday that the men would be tried on Saudi soil.