Turkish prosecutors have formally charged 20 Saudi nationals over the murder of the dissident journalist Jamal Khashoggi in the kingdom’s consulate in Istanbul in 2018, including two men close to the crown prince, Mohammed bin Salman.

The former royal court adviser Saud al-Qahtani and former deputy head of intelligence Ahmed al-Asiri were among the suspects charged with “instigating a premeditated murder with the intent of [causing] torment through fiendish instinct”, the office of the Istanbul chief prosecutor, Irfan Fidan, said on Wednesday. Assisted by three intelligence officers, a team of 15 men then travelled to Istanbul and carried out Qatani and Asiri’s orders, the statement said.

The gruesome killing stunned Saudi Arabia’s western allies, plunging the kingdom into its worst diplomatic crisis since the 9/11 attacks. It also irreversibly tarnished Prince Mohammed’s image as a liberal reformer after questions were raised over how such an operation could have been carried out without his consent or knowledge.

The CIA, along with several western governments, eventually concluded that the crown prince had ordered Khashoggi’s assassination. The kingdom has denied such claims, instead blaming rogue agents who it says took a repatriation mission too far.

Arrest warrants for the suspects have already been issued but since none are in the country a trial in absentia will be opened at an unspecified date, the prosecutor has said. Turkey is seeking life imprisonment in all 20 cases.

Saudi Arabia has rejected Turkish calls for the suspects’ return to face trial in Turkey. In December last year a Saudi court sentenced five unidentified people to death over Khashoggi’s killing but in effect exonerated Prince Mohammed’s inner circle.

The Saudi prosecutors also ruled that there had been no premeditation to kill at the beginning of the repatriation mission, a finding at odds with a UN inquiry published in June 2019 and now the Turkish indictment.

Agnès Callamard, the UN special rapporteur who authored the inquiry into Khashoggi’s death but was barred from access to the secretive Saudi trial, called the ruling a mockery of justice.

In response to international outrage over Khashoggi’s killing, King Salman ordered a shake-up of Saudi Arabia’s security apparatus, in which Qahtani and Asiri were dismissed from their positions. Asiri was subsequently tried but released because of insufficient evidence and Qahtani was investigated but not charged. He reportedly remains active in state matters.

Khashoggi, who was himself a member of the Saudi elite, broke with the royal family and moved to the US in 2017, where he became a vocal critic of the country as a columnist for the Washington Post. He visited the Saudi consulate in Istanbul on 2 October 2018 to pick up paperwork for his forthcoming marriage to his Turkish fiancee, Hatice Cengiz, and was never seen again.

The 59-year-old’s body has not been found. According to Wednesday’s indictment, based on analysis of phone and computer records, witness statements and the suspects’ entry into and exit from Turkey, investigators concluded that Khashoggi was strangled to death.

Turkish investigators have also alleged that Khashoggi’s body was dismembered with a bone saw and then dissolved in acid on the consulate premises.

Ankara used the killing to exert pressure on its Saudi regional rivals, drip-feeding lurid details to the media and sharing damning audio recordings of the murder with other governments.

Riyadh initially denied it had anything to do with Khashoggi’s disappearance. However, after sustained leaks from Turkish intelligence that suggested high-level Saudi involvement, the kingdom eventually admitted government agents had carried out the killing, offering a series of shifting explanations.

Callamard and several rights groups have repeatedly called for an independent international investigation into the journalist’s death.