Turkish authorities believe that prominent Saudi journalist Jamal Khashoggi, who disappeared four days ago after entering Saudi Arabia's consulate in Istanbul, was killed inside the consulate, two Turkish sources said on Saturday.

"The initial assessment of the Turkish police is that Mr. Khashoggi has been killed at the consulate of Saudi Arabia in Istanbul. We believe that the murder was premeditated and the body was subsequently moved out of the consulate," one of the sources, a Turkish official, told Reuters.

Khashoggi was "brutally tortured, killed and cut into pieces, everything was videotaped to prove the mission had been accomplished and the tape was taken out of the country," a senior Turkish police source told Middle East Eye.

State-funded broadcaster Al-Jazeera claimed that Saudi journalist Khashoggi's body was found in Istanbul region, but the information was not confirmed.

Khashoggi, who has lived in self-imposed exile in the U.S. for the last year, vanished Tuesday while on a visit to the consulate. His disappearance has threatened to upend already-fraught relations between Saudi Arabia and Turkey, and it raises new questions about the kingdom and the actions of its assertive Crown Prince Mohammed bin Salman, whom Khashoggi wrote about in his columns.

"If the reports of Jamal's murder are true, it is a monstrous and unfathomable act," the Post's editorial page editor Fred Hiatt said in a statement. "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."

The Post cited one anonymous official who said investigators believe a 15-member team "came from Saudi Arabia." The official added: "It was a preplanned murder."

A Turkish official, requesting anonymity to discuss the ongoing investigation, told The Associated Press earlier Saturday night something similar.

Khashoggi, 59, went missing while on a visit to the consulate in Istanbul for paperwork to marry his Turkish fiancée. The consulate insists the writer left its premises, contradicting Turkish officials.

"Jamal is not dead! I don't believe he's been killed!" his fiancée Hatice wrote on Twitter late Saturday night.

Anadolu Agency said Saturday that the Istanbul public prosecutor's office began a probe into Khashoggi's disappearance Tuesday, immediately after he went missing. It added the investigation over allegations that the writer was detained had "deepened," without elaborating.

Khashoggi is a longtime Saudi journalist, foreign correspondent, editor and columnist whose work has been controversial in the past in the ultraconservative Sunni kingdom. He went into self-imposed exile in the United States following the ascension of Prince Mohammed, now next in line to succeed his father, the 82-year-old King Salman.

As a contributor to the Post, Khashoggi has written extensively about Saudi Arabia, including criticizing its war in Yemen, its recent diplomatic spat with Canada and its arrest of women's rights activists after the lifting of a ban on women driving. All those issues have been viewed as being pushed by Prince Mohammed, who similarly has led roundups of activists, businessmen and others in the kingdom.

"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."

Khashoggi was known for his interviews and travels with Osama bin Laden between 1987 and 1995, including in Afghanistan, where he wrote about the battle against the Soviet occupation. In the early 1990s, he tried to persuade bin Laden to reconcile with the Saudi royal family and return home from his base in Sudan, but the al-Qaida leader refused.

Khashoggi maintained ties with Saudi elites, including those in its intelligence apparatus, and launched a satellite news channel, Al-Arab, from Bahrain in 2015 with the backing of Saudi billionaire Prince Alwaleed bin Talal. The channel was on air for less than 11 hours before it was shut down. Its billionaire backer was detained in the Ritz Carlton roundup overseen by Prince Mohammed in 2017.

Press freedom groups have decried Khashoggi's disappearance.