ISTANBUL -- A Turkish prosecutor has demanded that arrest warrants be issued against two Saudi nationals close to Crown Prince Mohammed bin Salman over the murder of journalist Jamal Khashoggi, a Turkish source close to the investigation said Wednesday.

Khashoggi, 59, was killed shortly after entering the Saudi consulate in Istanbul on October 2 to obtain paperwork for his upcoming marriage.

The chief prosecutor's office in Istanbul filed an application Tuesday to obtain the warrants for Ahmad al-Assiri and Saud al-Qahtani, described in court documents as being "among the planners" of the murder of the Washington Post contributor Khashoggi.

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Assiri often sat in during Prince Mohammed's closed-door meetings with visiting foreign dignitaries and Qahtani was a key counsellor to the crown prince. Both were sacked after Riyadh admitted Khashoggi was killed in the Saudi consulate.

According to Turkey, a 15-member Saudi team was sent to Istanbul to kill Khashoggi.

Turkish President Recep Tayyip Erdogan has said the order to kill Khashoggi came from the highest levels of the Saudi government, but has insisted it was not King Salman.

Riyadh has since detained 21 people over the murder. Despite speculation that the powerful crown prince ordered the hit, the kingdom has strongly denied he was involved.

Sen. Graham: "There's a smoking saw"

Mr. Trump has repeatedly defended the senior Saudi royals in the wake of the Khashoggi killing. In November, the president said bin Salman may have had no knowledge of the killing, and in any event the U.S. relationship with Saudi Arabia should not be jeopardized over it.

But two key U.S. Republican senators said a Tuesday briefing by the CIA's director only strengthened their conviction that Prince Mohammed directed the murder.

"There's not a smoking gun — there's a smoking saw," Republican Sen. Lindsey Graham told reporters after the classified meeting.

"I went into the briefing believing it was virtually impossible for an operation like this to be carried out without the crown prince's knowledge," Graham added. "I left the briefing with high confidence that my initial assessment of the situation was correct."

Sen. Bob Corker, an outspoken Republican critic of President Trump and his response to Khashoggi's killing, stressed, "If the crown prince went in front of a jury, he would be convicted in 30 minutes." Asked by a reporter if it would be a murder conviction, Corker replied "yes."

"All evidence leads back to the crown prince" Republican Sen. Richard Shelby told reporters as he left the Tuesday briefing. Two senators present told CBS News that Haspel did not share with lawmakers the audio tapes from the day Khashoggi was killed and reportedly dismembered inside the Saudi consulate. Secretary of Defense James Mattis has said the U.S. does not have any such tapes in its possession.

Unanswered questions

The Istanbul prosecutor in charge of the investigation said in late October that the Saudi former insider turned critic was strangled then his body was cut into pieces.

The remains of Khashoggi's body have not been found.

There has been speculation in pro-government media that his body was dissolved in acid.

A senior Turkish official Wednesday said the prosecutor's move "reflects the view that the Saudi authorities won't take formal action against those individuals".

The official, who did not wish to be named, pointed to the fact that the wording of the prosecutor's request suggested that the current list wasn't necessarily exhaustive, appearing to indicate that more arrest warrants could be sought.

Amid criticism from Ankara over Saudi Arabia's lack of cooperation with the Turkish investigation, the official said Riyadh could "address those concerns" over its commitment to probing the murder by extraditing all the suspects to Turkey.