A prominent Saudi journalist who contributed to The Washington Post was killed in "a pre-planned murder" at the Saudi Arabia consulate in Istanbul, Turkish investigators told the Post.

A critic of the kingdom and its Crown Prince Mohammed bin Salman, Jamal Khashoggi has been missing since he went to the consulate Tuesday to complete paperwork to be married to his Turkish fiancée, according to the Post.

Turkish investigators told the Post they think Khashoggi was killed by a 15-member Saudi team sent "specifically for the murder," according to the Post, which cited two people with knowledge about the investigation.

A Turkish official also told The Associated Press that detectives’ “initial assessment” was that Khashoggi was killed at the consulate, without elaborating. Saudi authorities early Sunday called the allegation “baseless.”

Also Sunday, President Recep Tayyip Erdogan called Khashoggi's disappearance "very, very upsetting," but would not confirm reports that he was killed at the consulate, the Post reported.

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Khashoggi, 59, who has lived in self-imposed exile in the U.S. for the last year, first visited the consulate on Sept. 28 to get a document related to his upcoming wedding, his fiancee and friends told the Post.

Prior to returning to the consulate on Tuesday, he was concerned he might not be allowed to leave, his fiancee Hatice Cengiz told the Post.

While Turkish authorities have said that Khashoggi never left the consulate, Saudi Arabia has denied Khashogg was detained there.

The Istanbul public prosecutor’s office began a probe into Khashoggi’s disappearance Tuesday, Turkey’s official Anadolu News Agency reported Saturday.

U.S. officials are aware of The Post's report about Khashoggi but cannot confirm it, and do not know where Khashoggi is, a senior U.S. official told the Post on Saturday.

“If the reports of Jamal’s murder are true, it is a monstrous and unfathomable act,” Fred Hiatt, director of The Post’s editorial page, said in a statement in a story reported by the Post. “Jamal was — or, as we hope, is — a committed, courageous journalist. He writes out of a sense of love for his country and deep faith in human dignity and freedom. He is respected in his country, in the Middle East and throughout the world. We have been enormously proud to publish his writings.”

Sen. Chris Murphy, D-Connecticut, a member of the Senate Committee on Foreign Affairs, said on Twitter the situation could affect relations between the two countries.

“If this is true — that the Saudis lured a U.S. resident into their consulate and murdered him — it should represent a fundamental break in our relationship with Saudi Arabia,” Murphy wrote on Twitter.

The Post printed a blank column in its Friday edition to bring attention to Khashoggi, who was a contributor to The Washington Post’s Global Opinions section.

Since he began contributing to The Post last year, Khashoggi has written on Saudi Arabia's war in Yemen, the kingdom's recent diplomatic spat with Canada and its arrest of women’s rights activists after the lifting of a ban on women driving, among other topics.

“With young Crown Prince Mohammed bin Salman’s rise to power, he promised an embrace of social and economic reform,” Khashoggi wrote in his first column for the Post. “But all I see now is the recent wave of arrests.”

Contributing: The Associated Press

Follow USA TODAY reporter Mike Snider on Twitter: @MikeSnider.

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