Turkish prosecutors have formally charged 20 suspects involved in the killing of Saudi journalist Jamal Khashoggi.

In a statement, Istanbul Chief Public Prosecutor İrfan Fidan said the investigation into the Saudi journalist’s murder has concluded. Turkey carried out its own investigation after dissatisfaction about Saudi Arabia's vague explanations and controversial statements.

The statement noted that the indictment accuses the former deputy head of Saudi Arabia's general intelligence, Ahmed al-Asiri, and former royal court adviser Saud al-Qahtani as having "instigated premeditated murder with monstrous intent."

It accuses 18 others of carrying out the killing of Khashoggi, a U.S. resident and columnist for The Washington Post, the prosecutor's office said in a statement.

All suspects, however, have left Turkey, and Saudi Arabia has rejected Turkish calls for their return to face trial in Turkey. Riyadh insisted the kingdom’s courts are the correct place for them to be tried and has put 11 people on trial over the killing.

Eighteen other suspects – including intelligence operative Maher Mutreb who frequently traveled with the crown prince on foreign tours, forensic expert Salah al-Tubaigy, and Fahad al-Balawi, a member of the Saudi royal guard – were also charged with "deliberately and monstrously killing, causing torment."

They face life in prison if convicted.

Mutreb, Tubaigy and Balawi had been among 11 people on trial in Riyadh. Western officials said many of those accused defended themselves by saying they were carrying out Assiri's orders, describing him as the operation's ringleader.

Five unnamed people were sentenced to death in Saudi Arabia in December while three others were handed jail terms totaling 24 years over the killing.

Qahtani had been investigated but he was not charged by the Saudi authorities because of "insufficient evidence" while Assiri was charged but eventually acquitted on the same grounds.

The Turkish prosecutor said a trial in absentia would be opened against the 20 suspects but did not give a date.

The prosecutor has already issued arrest warrants for the suspects, who are not in Turkey.

The indictment was based on analysis of mobile phone records of the suspects, records of their entry to and exit from Turkey, records of their presence at the consulate, witness statements and analysis of Khashoggi's phone, laptop and iPad, the statement said.

"In summary, the phone conversation records of the deceased, plaintiff and the suspects have been examined, and the footage of the deceased’s entry into the consulate, the suspects’ arrival in Turkey and their journey to the consulate building and the residence of the consul general have been acquired," the statement read.

A Red Notice has been issued by Interpol for the 20 suspects involved in the murder, while the indictment asks Saudi authorities to extradite the suspects.

Meanwhile, the U.N. human rights investigator who led the international probe into the murder of Khashoggi welcomed Turkey's indictment of the 20 Saudi nationals, saying it was needed as a "counterbalance to the travesty of justice at the hands of Saudi Arabia."

In a statement to Reuters, Agnes Callamard, U.N. rapporteur on extrajudicial killings, urged U.S. authorities to release their findings on responsibility for the death of The Washington Post columnist in October 2018, "including the responsibility of Saudi Arabia Crown Prince Mohammed Bin Salman."

Khashoggi was killed and dismembered by a group of Saudi operatives in the country's consulate in Istanbul on Oct. 2, 2018. Initially denying and later downplaying the incident as an accidental killing in a fistfight, Riyadh finally admitted almost three weeks after the disappearance that Khashoggi was murdered in a premeditated fashion but denied any involvement of the royal family.

The incident was blamed on lower-level officials, including five who are now facing the death penalty over their involvement. A Saudi public prosecutor said in late March that they would seek the death penalty for five suspects among the 21 involved in the case. Ankara has said the statement is not satisfactory and demanded genuine cooperation from Riyadh.

To this day, Khashoggi's body has not been recovered, and the kingdom has remained silent on its whereabouts.