Jamal Khashoggi, a critic of Riyadh, had gone to the building in Istanbul seeking documents for his marriage

Turkish officials believe that missing Saudi journalist Jamal Khashoggi was killed inside the Saudi consulate in Istanbul and his body later driven from the compound.

Authorities say they believe Khashoggi’s death was premeditated and that Saudi officials had travelled to Istanbul from Riyadh after receiving word that the high-profile critic of the current Saudi leadership planned to visit the consulate.

In an evening of quickfire developments, following four days of silence since his disappearance, officials in Ankara pledged to on Sunday release evidence that they say supports claims that the journalist was killed shortly after he entered the consulate to sign divorce papers. The evidence is expected to include video footage and focus on a black car.

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Two Turkish officials claimed to Reuters that Khashoggi, 59, had been killed. The Reuters claim was circulated by a government spokesman, and confirmed by numerous other officials, some of whom claimed to have knowledge of how the body had been disposed of. Several officials alleged, without tabling evidence, that Khashoggi had first been tortured.

Officials believe that a team of 15 Saudis arrived on Tuesday to conduct the killing, then left the country soon afterwards.

The president, Recep Tayyip Erdoğan, is expected to release a statement about the incident on Sunday. Aside from summonsing the Saudi ambassador in Ankara, senior officials had remained mute about Khashoggi’s fate, leading to speculation that he had been smuggled out of the country with Turkish consent.

The dramatic Turkish claim instead squarely focuses attentions on Riyadh, in particular Crown Prince Mohammed bin Salman, who on Friday denied any knowledge of Khashoggi’s whereabouts. “If he was here, I would know about it,” the 33-year-old heir to the throne told Bloomberg.

“My understanding is, he entered and he got out after a few minutes or one hour,” said Prince Mohammed. “I’m not sure. We are investigating this through the foreign ministry to see exactly what happened at that time.

“We are ready to welcome the Turkish government to go and search our premises. The premises are sovereign territory, but we will allow them to enter and search and do whatever they want to do. If they ask for that, of course, we will allow them.”

Facebook Twitter Pinterest Protesters hold up pictures of Jamal Khashoggi outside the Saudi Arabian consulate in Istanbul. Photograph: Chris McGrath/Getty Images

Earlier on Saturday, a Saudi official had dismissed claims that Khashoggi was still inside the consulate and said authorities were “very concerned for his safety”.

Khashoggi had been close to the previous Saudi regime, but fled the kingdom a year ago, soon after the new monarch anointed his son, Prince Mohammed, as prince. He subsequently became an outspoken critic of some aspects of the country’s reform programme, especially the clampdown on political freedoms, and intolerance of dissent.

Khashoggi had a regular column for the Washington Post and had also written for the Guardian. He had sought assurances about his safety from Saudi officials before entering the consulate to sign the divorce papers, a necessary step for him to marry his Turkish fiance, who was waiting for him outside.