Saudi Arabia is preparing to acknowledge the death of Saudi journalist Jamal Khashoggi happened as the result of a botched interrogation, according to CNN and The New York Times, after US President Donald Trump speculated "rogue killers" may be responsible.

Key points: Mr Trump says Mr Pompeo may also visit Turkey during the trip

Mr Trump says Mr Pompeo may also visit Turkey during the trip He says it sounded like "rogue killers" were involved in Khashoggi's disappearance

He says it sounded like "rogue killers" were involved in Khashoggi's disappearance The US President says Saudi monarch King Salman has "denied any knowledge"

Khashoggi, a US resident, Washington Post columnist and leading critic of the powerful Saudi crown prince, Mohammed bin Salman, vanished after entering the Saudi consulate in Istanbul two weeks ago to get marriage documents.

Turkish officials said they believe he was murdered there and his body removed, in a case that has provoked an international outcry.

Members of Khashoggi's family called for an investigation in a statement released on Monday.

"We are sadly and anxiously following the conflicting news regarding the fate of our father after losing contact with him two weeks ago," they said.

"The strong moral and legal responsibility which our father instilled in us obliges us to call for the establishment of an independent and impartial international commission to inquire into the circumstances of his death."

Citing two unidentified sources, CNN reported that Saudi Arabia was preparing a report that would acknowledge Khashoggi was killed as the result of an interrogation that went wrong.

The Saudi Government could not be reached immediately for comment on the CNN report, but Mr Trump said he was unsure whether it was official.

"I heard that report, but nobody knows if it's an official report," Mr Trump said, without elaborating, while visiting Florida to inspect the damage from Hurricane Michael.

Sorry, this video has expired Turkish officials say they fear a Saudi assassination team killed Khashoggi.

The New York Times, citing a person familiar with the Saudi plans, reported the crown prince had approved an interrogation or rendition of Khashoggi back to Saudi Arabia.

The Saudi Government, it said, would shield the prince by blaming an intelligence official for the bungled operation.

Turkish authorities have an audio recording indicating that Khashoggi was killed in the consulate, a Turkish official and a security source said, and have shared evidence with countries including Saudi Arabia and the United States. They provided no further details.

Saudi Arabia has strongly denied killing Khashoggi and has denounced such assertions as "lies", saying he left the building shortly after entering.

'Maybe these could have been rogue killers'

Earlier, Mr Trump proposed that Khashoggi may have been killed by "rogue killers" as he sent Secretary of State Mike Pompeo to meet with Saudi Arabia's King Salman.

Mr Trump said King Salman had told him he had no knowledge of Khashoggi's fate.

"The King firmly denied any knowledge of it," Mr Trump said.

"He didn't really know, maybe — I don't want to get into his mind but it sounded to me — maybe these could have been rogue killers. Who knows?"

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State Department spokeswoman Heather Nauert said Mr Trump was sending Mr Pompeo to Riyadh because "determining what happened to Jamal Khashoggi is something of great importance to the President".

Mr Trump has said Saudi Arabia could face "severe punishment" if it was proven it was involved in Khashoggi's disappearance.

Jamal Khashoggi had written newspaper columns critical of Saudi Arabia's ruling prince. ( AP: Hasan Jamali, file )

US politicians have backed the call for tough punitive action against the Saudis, while Germany, France and Britain have jointly called for a "credible investigation" into his disappearance.

Saudi Arabia has warned that it would retaliate against any punishment with even greater measures.

Police investigators enter consulate

Mr Pompeo's visit comes as Turkish police investigators entered the Istanbul consulate late on Monday.

A Turkish diplomatic source earlier said a joint Turkish-Saudi team would search the building — the last place Khashoggi was seen before he vanished on October 2.

"It has been 13 days since the event, so surely proving some of the evidence might be difficult, but we believe we will obtain evidence," the Turkish official said.

According to a Reuters witness, a crime scene investigation team spent more than nine hours in the building before they left the consulate early on Tuesday.

Four forensic vehicles arrived outside the consulate and took away soil samples, as well as a metal door from the garden, the witness said.

A police dog was also part of the search team.

King Salman and Turkish President Tayyip Erdogan spoke by telephone on Sunday evening and stressed the importance of the two countries creating a joint group as part of the probe.

Turkish officials have said they fear a Saudi assassination team that flew into and out of Turkey on October 2 killed and dismembered Khashoggi.

The kingdom has called such allegations "baseless", but has not offered any evidence Khashoggi ever left the consulate.

King Salman ordered the public prosecutor to open an internal investigation into Khashoggi's disappearance.

ABC/wires