ISTANBUL — Prosecutors in Istanbul said Wednesday they had indicted 20 Saudi nationals, including two senior officials, on murder charges after an investigation into the 2018 killing of journalist Jamal Khashoggi, according to a statement from the prosecutor's office.

The suspects are all believed to be in Saudi Arabia, where prosecutors conducted a separate, closed-door trial for Khashoggi’s killing. Saudi officials had previously refused Turkey’s request to extradite Saudi suspects in the case.

Khashoggi, 59, a veteran journalist and columnist for The Washington Post, was killed in the Saudi Consulate in Istanbul in October 2018 by a team of agents dispatched to Turkey by senior Saudi officials, according to Turkish and Saudi prosecutors. After Khashoggi was strangled, his body was dismembered and his remains have yet to be recovered, Turkish officials said.

Before his death, Khashoggi, writing in The Post, had been critical of Crown Prince Mohammed bin Salman, Saudi Arabia’s day-to-day ruler.

On Oct. 2, 2018, Saudi agents killed Washington Post columnist Jamal Khashoggi inside the Saudi Consulate in Istanbul. What has been done in the aftermath? (Joyce Lee, Thomas LeGro, Dalton Bennett, John Parks/The Washington Post)

The killing shone a light on the extraordinary crackdown on dissent carried out by Saudi authorities under Mohammed’s leadership and soured relations between Saudi Arabia and Turkey. President Recep Tayyip Erdogan’s government first publicized details of the Saudi plot and has continued to accuse the Saudi leadership of shielding senior officials from blame.

The CIA concluded that Mohammed had probably ordered the killing, which the crown prince has denied.

The Turkish indictment was the latest sign that Erdogan’s government would continue to highlight Khashoggi’s case. It charged two top Saudi officials, Saud al-Qahtani, a close adviser to Mohammed, and Ahmed al-Assiri, a former deputy head of Saudi intelligence, with having “instigated premeditated murder with monstrous intent.”

The two men “gave the other suspects the necessary instructions to fulfill the action of killing” Khashoggi, the statement said.

Both men were cleared of any wrongdoing by Saudi prosecutors late last year. Five other unidentified men were sentenced to death in the trial, which was criticized for lacking transparency by human rights advocates.

In addition to Assiri and Qahtani, Istanbul prosecutors charged 18 other men in the killing, after investigators collected evidence from surveillance footage, phone logs, and Khashoggi’s laptop and cellphones, the statement said.